JP2011035020A - Bonding wire for semiconductor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bonding wire for semiconductor, which ensures good wedge bonding even of palladium-plated lead frames and has excellent oxidation resistance, and in which copper or a copper alloy is used as a core wire. <P>SOLUTION: The bonding wire for semiconductor is characterized by comprising a core wire that comprises: copper or a copper alloy; a coating layer that is arranged on the surface of the core wire, has a thickness of 10 to 200 nm and contains palladium; and an alloy layer that is arranged on the surface of the coating layer, has a thickness of 3 to 30 nm and contains silver and palladium, wherein the silver is contained in the alloy layer of the silver and palladium at a concentration of 10 to 70 vol% inclusive. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

  At present, a semiconductor bonding wire (hereinafter referred to as “bonding wire”) for connecting between an electrode on a semiconductor element and an external connection terminal has a wire diameter of about 20-50 μm and is made of high-purity 4N (4-Nine). Bonding wire (gold bonding wire) which is gold (Au) having a purity of 99.99% by mass or more is mainly used. In order to bond a gold bonding wire to an electrode on a silicon chip that is a semiconductor element, it is common to perform ball bonding by a thermocompression bonding method using ultrasonic waves. That is, using a general-purpose bonding apparatus, the gold bonding wire is passed through a jig called a capillary, the wire tip is heated and melted by arc heat input, and a ball portion is formed by surface tension. This is a technique in which a ball portion formed by heating and melting is pressure-bonded onto the electrode heated within a range of ° C.

  On the other hand, when connecting a gold bonding wire to an external connection terminal such as a lead or a land, so-called wedge bonding can be performed by directly bonding the gold bonding wire to the electrode without forming the ball portion as described above. It is common. In recent years, the structure, materials, connection technology, etc. of semiconductor mounting have been diversified rapidly. For example, in the mounting structure, in addition to QFP (Quad Flat Packaging) using the current lead frame, a substrate, polyimide tape, etc. are used. New mounting forms such as BGA (Ball Grid Array) and CSP (Chip Scale Packaging) have been put into practical use, and external connection terminals are also diversified. For this reason, wedge bonding characteristics are becoming more important than ever.

  In addition, there is a growing need for miniaturization of semiconductor elements, and in order to achieve thin mounting, low loop bonding technology that lowers the height of the bonding wire connection loop and loop from the substrate side toward multiple stacked chips The reverse-bonding technology that launches is spreading.

By the way, with the recent increase in resource prices, the price of gold as a raw material for gold bonding wires has also increased rapidly, and copper (Cu) has been studied as a low-cost wire material that can replace gold. However, since copper is more easily oxidized than gold, it is difficult to store for a long time with a simple copper bonding wire, and the wedge bonding characteristics are not good. Moreover, when forming a ball part at the tip of such a simple copper bonding wire, a reducing atmosphere must be provided so that the ball part is not oxidized. Specifically, it is common to use a gas in which about 4% by volume of hydrogen (H ₂ ) is mixed with nitrogen (N ₂ ) to create a reducing atmosphere around the ball part. It is difficult to perform good ball bonding using a bonding wire. For these reasons, the use of copper bonding wires has not yet spread to the general LSI field.

  Then, in order to solve the subject called oxidation of a copper bonding wire, the copper bonding wire which coat | covered silver (Ag) on the surface of a copper wire is proposed. For example, Patent Document 1 does not show a specific example in which a copper wire is coated with silver, but the inner metal of the bonding wire is aluminum (Al), copper, iron (Fe), an alloy of iron and nickel (FeNi), or the like. It is disclosed that the surface metal of the bonding wire is a metal having corrosion resistance against moisture, salt, alkali, etc., for example, gold or silver. Patent Document 2 does not show a specific example in which a copper wire is coated with silver, but a copper-based bonding wire in which a copper-based wire is coated with a noble metal including gold and silver is exemplified. It is described that if the coating is applied, the corrosion resistance is further improved. In Patent Document 3, a bonding wire obtained by plating a noble metal such as gold or silver on aluminum (Al) or copper wire is disclosed. In the case of a copper bonding wire, the problem of corrosion resistance and thermal oxidation is eliminated by the plating, The bondability with the lead frame is said to be as reliable as the gold bonding wire. Patent Document 4 discloses a copper bonding wire in which a surface of a high-purity copper fine wire is coated with a noble metal or a corrosion-resistant metal, and silver is used as one of the noble metals to be coated. By configuring in this way, the surface oxidation of the copper bonding wire (specifically, the presence or absence of surface oxidation after being left in the atmosphere for 10 days) can be suppressed. In addition, the diameter of the copper fine wire is 15 to 80 μm, and the coating film to be coated has an average layer thickness of 10 nm to 1 μm (in the example, a wire having a diameter of 25 μm and an average layer thickness of 0.1 μm). It is a coating.) In Patent Document 5, a copper bonding wire in which silver is coated to a thickness of 0.001 to 0.01 times the wire diameter on the surface of a copper fine wire, that is, a copper fine wire having a diameter of 25 μm and a thickness of 0.02 to 0.3 μm. A copper bonding wire to be a silver coating is disclosed. By covering silver, the oxidation of copper is suppressed and the ball forming ability is improved.

However, as described above, the copper bonding wire in which the wire surface is coated with silver can suppress copper surface oxidation (particularly the progress of oxidation during storage), but the ball part formed at the wire tip during bonding is true. In many cases, it does not become a sphere, but it becomes distorted, which hinders the practical application of the copper bonding wire. This is because when the wire tip is heated and melted by arc heat input, silver having a low melting point (melting point 961 ° C.) is preferentially melted, whereas copper having a high melting point (melting point 1083 ° C.) is only a part. It seems related to not melting. Further, as described in Patent Document 5, if bonding is performed in a reducing atmosphere (10% H ₂ —N ₂ ), ball formation is often good even with silver coating, but in an atmosphere that does not contain hydrogen, Therefore, it is difficult to perform bonding, and good ball portion formation cannot be achieved.

  On the other hand, instead of coating silver, it is conceivable to coat palladium (Pd) on the surface of the copper wire. Actually, Patent Documents 2 to 4 also exemplify palladium as a noble metal other than silver in the coating layer. The above document does not show the predominance of palladium, but when palladium having a melting point higher than silver (melting point: 1554 ° C.) is coated, the copper wire is melted like the above-mentioned silver before the ball part is formed. It is considered that the problem that the coating layer melts and a spherical ball portion cannot be formed can be solved. That is, it is considered that the two problems of preventing oxidation of copper and ensuring the sphericity of the ball part can be solved simultaneously by coating the surface of the copper wire with palladium. Patent Document 6 discloses that in a two-layer bonding wire of a core wire and a coating layer (outer peripheral portion), a diffusion layer is provided between the core wire and the coating layer to improve the adhesiveness of the coating layer. In this example, copper is used for the coating layer and palladium is used for the coating layer. In such a copper bonding wire coated with palladium, copper oxidation is suppressed, so that not only the copper bonding wire is stored for a long time and the wedge bonding characteristics are excellent, but also the ball portion is formed when the ball portion is formed at the tip of the wire. Concern about oxidation is greatly improved. Therefore, a true ball portion can be formed by using pure nitrogen gas without using hydrogen, which is a dangerous gas, and simply forming a nitrogen atmosphere around the ball portion.

JP 57-12543 A JP 59-181040 A JP-A 61-285743 JP-A-62-97360 JP 62-120057 A Republished WO2002-23618

  As mentioned above, copper bonding wire has become practical as a bonding wire that is cheaper than gold bonding wire by covering the surface of copper wire with palladium. The problem of not always being able to cope with rapid changes and diversification of connection technology has become apparent.

  For example, the surface of a lead frame so far is generally silver-plated, but recently, a lead frame plated with palladium has been used. This is because the conventional silver-plated lead frame (hereinafter referred to as “silver-plated lead frame”) is intended to increase the wettability with solder before soldering the lead frame to a substrate such as a mother board. Since there was a process to solder thinly on the tip of the lead in advance (solder plating process), it was expensive, so palladium that can secure higher wettability to solder than silver was put on the lead frame instead of silver By plating, the solder plating step is omitted and the cost is reduced.

  In the case of a copper bonding wire in which the surface of the copper wire is coated with palladium, the inventors have not made it obvious in the conventional silver-plated lead frame, but the wedge bondability to the palladium-plated lead frame becomes insufficient. I found the problem that there were many cases. Furthermore, the inventors have studied the above-mentioned problem in detail, and since the outermost surface of the copper bonding wire is palladium, palladium contacts with each other in wedge bonding to a lead frame plated with palladium. Then, since the hardness of palladium (palladium Mohs hardness 4.75, copper Mohs hardness 3.0) is high, palladium is not easily deformed, and therefore the destruction of the oxide film layer on the palladium surface is insufficient. We found that it was the cause of the above problem. Furthermore, it has been found that the fact that a sufficient diffusion layer is not formed between both palladium layers due to slow diffusion between the palladium on the outermost surface of the wire and the palladium on the lead frame is also the cause of the above problem. .

  The present invention has been made in view of the above-mentioned problems, and the object of the present invention is to secure good wedge bondability even with a palladium-plated lead frame and to have excellent oxidation resistance. Alternatively, it is an object to provide a bonding wire for a semiconductor element having a copper alloy as a core wire.

  The gist of the present invention for achieving the above-described object is as follows.

  A bonding wire for semiconductor according to claim 1 is a core wire made of copper or a copper alloy, a coating layer containing palladium formed on the surface of the core wire with a thickness of 10 to 200 nm, and a surface of the coating layer, An alloy layer of silver and palladium formed with a thickness of 3 to 30 nm, wherein the silver concentration in the alloy layer is 10% by volume to 70% by volume.

  The bonding wire for a semiconductor according to claim 2 has an area of a crystal grain in which the inclination of the <100> crystal orientation with respect to the drawing direction is 15 degrees or less among the surface crystal grains of the alloy layer is 50% or more and 100% or less It is characterized by being.

  The bonding wire for semiconductor according to claim 3 is characterized in that the Mayer hardness of the surface of the bonding wire is in a range of 0.2 to 2.0 GPa.

  The bonding wire for semiconductor according to claim 4 is characterized in that the concentration of silver in the alloy layer is 20% by volume or more and 70% by volume or less.

  The bonding wire for semiconductor according to claim 5 is characterized in that the core wire contains 5 to 300 mass ppm in total of at least one of B, P, and Se.

  ADVANTAGE OF THE INVENTION According to this invention, even if it is a palladium plated lead frame, favorable wedge bondability can be ensured, and it can provide the cheap bonding wire for semiconductor elements which made copper or copper alloy a core wire excellent in oxidation resistance. .

  Below, the structure of the bonding wire of this invention is further demonstrated. In the following description, “%” means “volume%” unless otherwise specified. In addition, the composition is an average value of only the metal values obtained when analyzing a plurality of locations, and carbon exists as a natural contaminant (inevitable impurity) but is not included in the composition described below. .

  An inexpensive bonding wire that secures both good wedge-bonding and oxidation resistance on a palladium-plated lead frame (hereinafter referred to as “palladium-plated lead frame”) and uses copper or copper alloy as the core wire. For providing, a coating layer containing palladium having a specific thickness is formed on the surface of a core wire made of copper or a copper alloy, and the surface of the coating layer is made of an alloy of silver and palladium having a specific thickness and a specific composition. The present inventors have found that the bonding wire is effective.

First, the structure which forms the coating layer containing palladium of appropriate thickness on the surface of the core wire which consists of copper or a copper alloy is demonstrated. As mentioned above, copper or copper alloy is easily oxidized, so long as the bonding wire made of copper or copper alloy is inferior in long-term storage and wedge bonding characteristics, a coating layer containing palladium is formed on the surface of the core wire made of copper or copper alloy. If this is done, the copper oxidation is suppressed, which not only improves the long-term storage and wedge bonding characteristics described above, but also greatly increases the concern that the ball part will oxidize when the ball part is formed at the tip of the bonding wire. It will be improved. This is because the coating layer contains palladium that is not easily oxidized compared to copper (that is, the oxide generation heat ΔH ₀ is large), and thus the above-described effect can be obtained. Therefore, a true ball portion can be formed by using pure nitrogen gas and making the surrounding of the ball portion a nitrogen atmosphere without using a gas mixture of hydrogen and nitrogen, which is a dangerous gas. In order to obtain such an effect, the thickness of the coating layer needs to be 10 to 200 nm, and if it is less than 10 nm, the oxidation suppressing effect is insufficient. When the thickness of the coating layer exceeds 200 nm, bubbles with a diameter of several μm are often generated on the surface of the ball part, which is not preferable. Here, the element contained other than palladium in the coating layer containing palladium is an element constituting the inevitable impurity of palladium and the outermost surface of the core wire or wire. Further, if the content of palladium in the coating layer is 50% or more, a sufficient oxidation suppressing effect can be obtained. However, as an element other than palladium contained in the coating layer, silver constituting the outermost surface described later is not contained, or when silver is contained, the silver concentration is preferably less than 10%. This is because when the silver concentration of the coating layer is 10% or more, the above-described problems of the silver-coated wire (oxidation during ball formation, etc.) appear.

  Only the above-described configuration in which the coating layer containing palladium is formed on the surface of the core wire made of copper or a copper alloy cannot ensure good wedge bondability on the palladium-plated lead frame. In order to solve this problem, the present inventors have found that an alloy layer of silver and palladium may be further formed on the surface of the coating layer. The alloy layer is further formed with a thickness of 3 to 30 nm on the coating layer. This is due to the fact that the wedge bondability is governed by the physical property values in the region of about 3 nm from the outermost surface of the wire. In other words, if the region of 3 nm from the outermost surface of the wire is an alloy of silver and palladium, the silver in the alloy of silver and palladium constituting the outermost surface of the wire when wedge-bonded onto the palladium plating lead frame Diffuses preferentially toward the palladium on the palladium plated lead frame, making it easier to form an alloy layer between the bonding wire and the palladium plated lead frame. Therefore, the wedge bonding property with the palladium plating lead frame is improved, and for example, the 2nd peel strength is improved. This is due to the fact that interdiffusion between silver and palladium is faster than self-diffusion of palladium. However, if the thickness of the alloy layer is less than 3 nm, the covering layer, which is the base of the bonding wire, affects the wedge bonding property, so that the wedge bonding property with the palladium plated lead frame cannot be ensured. In order to obtain the effect, there is no particular limitation on the upper limit of the thickness of the alloy layer of silver and palladium, but in order to make the thickness more than 30 nm, the furnace temperature to be described later must be higher than 720 ° C. Since it is difficult to ensure stable quality, the upper limit of the thickness of the alloy is set to 30 nm or less.

  In addition, in order to obtain the above-described effect by the alloy layer of silver and palladium, the silver composition (silver concentration) in the alloy layer needs to be in a specific range. Specifically, if the silver concentration in the alloy layer of silver and palladium is 10% or more and 70% or less, and more preferably 20% or more and 70% or less, wedge bonding with the above-described palladium plating lead frame It is good because the nature is further enhanced. If the silver concentration is less than 10%, the above-mentioned effects cannot be obtained. Conversely, when the silver concentration exceeds 70%, when the ball portion is formed at the tip of the wire, only the silver in the alloy layer made of silver and palladium is preferentially melted to form a distorted ball portion. Not good because it increases the risk. On the other hand, if the silver concentration in the alloy layer is 70% or less, since silver and palladium are homogeneously mixed in the surface layer of the wire, only silver is preferentially melted when forming the ball portion at the wire tip. Thus, there is no risk that an irregular ball portion is formed, and the sphericity and dimensional accuracy of the ball portion are not impaired. Further, if the silver concentration is 10% or more and 40% or less, the sphericity and dimensional accuracy of the ball part may be further improved.

  Therefore, in a bonding wire in which a coating layer containing palladium having an appropriate thickness is formed on the surface of a core wire made of copper or a copper alloy, and an alloy layer of silver and palladium having an appropriate thickness and composition is applied to the surface of the coating layer. Therefore, it is possible to provide an inexpensive bonding wire having both good wedge bondability and oxidation resistance on the palladium plated lead frame and having copper or a copper alloy as a core wire.

  Further, it has been found that the following effects can be obtained at the same time when the silver concentration in the alloy layer of silver and palladium is 20% or more and 70% or less.

  In general, in the region where the capillary and the bonding wire are in contact with each other on the inner wall of the capillary, the capillary and the bonding wire are constantly rubbed during the bonding process. Therefore, the inner wall of the capillary is processed so that there is no unevenness in the region.

  Incidentally, in the conventional case, when a bonding wire having only a covering layer containing palladium on the surface of a core wire made of copper or a copper alloy is used and a long span bonding exceeding, for example, 5 mm is repeated many times, the capillary and the bonding are bonded. The area of the capillary that comes into contact with the wire wears out. Then, sharp unevenness is generated in the region, and as a result, the scratch formed by the capillary becomes conspicuous on the wire surface. This is because palladium is a hard metal, and the coating layer containing palladium is also hard.

  On the other hand, in the present invention, in the alloy layer of silver and palladium provided on the surface of the coating layer, the concentration of silver in the alloy layer is increased. Can be suppressed. In the above alloy layer of silver and palladium, silver is homogeneously mixed so as to be called a solid solution with palladium, and when the concentration of silver is high, silver is preferential in the region where the capillary and the bonding wire are in contact with each other. By contributing to the deformation, it is possible to suppress the occurrence of such sharp irregularities. Such an effect can be obtained when the silver concentration is 20% or more, more preferably 30% or more. On the other hand, if the silver concentration is 70% or more, the sphericity and dimensional accuracy of the ball portion cannot be sufficiently obtained for the reasons described above.

  Further, it has been found that the following effects can be obtained at the same time when the silver concentration in the alloy layer is 20% or more in the alloy layer of silver and palladium.

  Incidentally, in the conventional case, in a bonding wire having only a coating layer containing palladium on the surface of a core wire made of copper or a copper alloy, if a ball part having a diameter of 30 μm or more is formed at the tip of the bonding wire, a bubble having a diameter of several μm May occur frequently on the surface of the ball portion. This is related to the recent downsizing and higher functionality of electronic devices. In other words, in order to support miniaturization and high functionality of electronic devices, semiconductor elements are also miniaturized and highly functional, but in the case of bonding wires, a ball formed at the tip of the wire in order to reduce the area of the joint. There is an increasing tendency to reduce the size of the ball, and in the past, a ball having a diameter of less than 50 μm was used, but a ball having a diameter of more than 30 μm has been used in mass production. Although the above-mentioned minute bubbles of several μm have been formed even in the conventional ball portion having a diameter of 50 μm or more, the joining area is inevitably increased due to the large ball diameter. Bubbles have not been particularly problematic until now. However, since the joint area is small in the ball part with a diameter of slightly more than 30 μm these days, even the above-mentioned bubbles that have not been a problem until now can improve the joint strength and long-term reliability of the joint part. It is believed to have an impact and is becoming a problem.

  The present inventors have found that the location of such bubbles is always palladium. In other words, the cause of the bubbles is that when the ball portion is formed, palladium existing on the wire surface segregates in the ball to form a concentrated region of the palladium single layer, and organic matter-derived gas is confined in the region. It is in.

  On the other hand, in the present invention, since the surface of the coating layer containing palladium contains silver having a specific concentration or more, when the ball portion is formed, a palladium concentrated region is not formed, but instead silver. -A concentrated region of palladium alloy or copper-palladium-silver ternary alloy will be formed. Therefore, in the bonding wire of the present invention, if it is the concentrated region, the risk of trapping gas due to organic matter is lessened. Therefore, even when a ball part having a small diameter of slightly over 30 μm is formed, generation of bubbles is prevented. Can be suppressed. That is, when the silver concentration in the alloy of silver and palladium according to the present invention is 20% or more, the above effect can be obtained, and more preferably 30% or more, the effect is further enhanced. So good.

  As a method for measuring the thickness and composition of the coating layer and the alloy layer, a method of analyzing the surface of the bonding wire while digging in the depth direction by a sputtering method, or a line analysis or a point analysis in a cross section of the bonding wire is effective. In the former method of measuring while digging, if the measurement depth increases, it takes too much measurement time. The latter line analysis or point analysis is advantageous in that it is relatively easy to confirm the concentration distribution over the entire cross section, reproducibility at several locations, and the like. Line analysis is relatively simple in the cross section of the bonding wire, but if you want to improve the accuracy of the analysis, narrow the analysis interval in the line analysis or expand the area to be analyzed in detail, and then perform point analysis. It is also effective to perform. Here, the thickness of the alloy layer is the distance (depth) of the portion where the silver concentration is 10% by volume or more by analyzing the composition in the depth direction from the surface. The thickness of the coating layer is the distance (depth) of the portion where the concentration of palladium is 50% or more by composition analysis in the depth direction from the interface that becomes the thickness of the alloy layer. EPMA (Electron Probe Micro Analysis), EDX (Energy Dispersive X-Ray Analysis), AES (Auger Electron Spectroscopy) are used as analytical equipment for these analyses. TEM (Transmission Electron Microscope) can be used. If the thickness and composition obtained by any one of the above methods are within the scope of the present invention, the effects of the present invention can be obtained.

  In order to ensure both good wedge bondability and oxidation resistance on the palladium-plated lead frame as described above, and to satisfy the loop characteristics described later, the crystal orientation of the wire surface and the hardness of the wire surface In addition, the inventors have found that a bonding wire having a specific range of the type and composition of the additive element in the core wire is effective.

  As for the crystal orientation of the wire surface, it is more preferable that the <100> crystal orientation has no or little inclination with respect to the drawing direction among the surface crystal grains of the alloy layer. Specifically, if the area of the crystal grains having the inclination of 15 degrees or less is set to 50% or more and 100% or less, wrinkles are unlikely to occur on the surface of the loop even when reverse bonding is performed. More preferably, it is 70% or more and 100% or less because the effect is further enhanced. Here, the wrinkle is a general term for minute scratches and irregularities on the surface that are generated when a loop is formed. As a result, for example, ball bonding is performed on the 2nd bonding electrode, which is increasing recently, and wedge bonding is performed on the 1st bonding electrode, thereby suppressing the loop height and facilitating the thinning of the chip.

  By the way, in the reverse bonding as described above, first, the ball is bonded to the 1st bonding electrode, the bonding wire immediately above the bonded ball is cut, and then the ball bonding is performed to the 2nd bonding electrode. Wedge bonding is performed on the ball portion on the first bonding electrode. At this time, when the bonding wire on the ball is cut after bonding the ball to the 1st bonding electrode, if a large impact is applied to the bonding wire, the surface of the bonding wire will be wrinkled. When thermal fatigue of cooling to room temperature accompanying stoppage is applied to the device for a long period of time, the wrinkles may accelerate the generation of cracks.

  As a result of intensive studies by the present inventors, this wrinkle defect is related to the crystal orientation of the wire surface (alloy layer), and the strength is high but ductile as represented by <111> crystal orientation. It was found that wrinkles were noticeably generated when the orientation was poor. As a result of further studies by the present inventors, in order to suppress the wrinkle, the inclination of the <100> crystal orientation with respect to the wire drawing direction is reduced on the wire surface, and the crystal grains having the inclination of 15 degrees or less are reduced. It was found that when the area was 50% or more, ductility sufficient to suppress wrinkles could be secured. However, such an effect cannot be obtained when the area of the crystal grains having the inclination of 15 degrees or less is less than 50%. Here, the inclination of the <100> crystal orientation of the crystal grain observed on the surface of the alloy layer with respect to the drawing direction is determined by a micro-region X-ray method or an electron backscattered pattern (EBSD, Electron Backscattered) installed in a TEM observation apparatus. Diffraction) method or the like. Among other things, the EBSD method has the feature that the orientation of individual crystal grains can be observed and the angle difference of the crystal orientation between adjacent measurement points can be illustrated, and even a thin wire such as a bonding wire is relatively simple. However, it is more preferable because the inclination of the crystal grains can be observed with high accuracy. In addition, the area of a crystal grain having an inclination of 15 degrees or less can be obtained as a volume ratio of the crystal orientation based on the X-ray intensity of the crystal orientation in each crystal grain in the micro region X-ray method. In the method, it is possible to directly calculate from the orientation of the individual crystal grains observed as described above. In order to calculate the ratio of the area, the width of at least 1/4 of the diameter of the bonding wire in the direction perpendicular to the wire drawing direction of the bonding wire is an arbitrary surface of the wire surface, and the wire drawing direction of the bonding wire A surface having a length of at least 100 μm is observed, and the observation area is defined as 100, and the percentage of the area occupied by the crystal grains having the inclination of 15 degrees or less is defined. If the thickness and composition obtained by any one of the above methods are within the scope of the present invention, the effects of the present invention can be obtained.

  Regarding the hardness of the wire surface, if the Meyer hardness of the wire surface is in the range of 0.2 to 2.0 GPa, the occurrence of defects called neck damage is suppressed even during low loop bonding with a loop height of 80 μm class. So even better.

  This neck damage refers to damage in the boundary area (neck part) between the ball part and the busbar part. It is a defect that occurs when an excessive load is applied to the neck part when forming a loop with an extremely low loop height. is there. In recent thin electronic devices such as flash memory, thin devices with multiple thin silicon chips are used in order to increase the capacity of the memory as much as possible. Since the loop height must be lowered, the neck damage has been easily generated.

  The inventors clarified that the hardness of the wire surface is closely related to the occurrence of the neck damage, and if the hardness is lowered, even if an excessive load is applied to the neck during low-loop bonding. It was found that the surface can be plastically deformed and neck damage can be suppressed. Specifically, the above effect can be obtained by setting the Meyer hardness of the surface of the bonding wire to 2.0 GPa or less. However, when the Meyer hardness of the surface of the bonding wire exceeds 2.0 GPa, the hardness becomes the same as that of a normal silver alloy, and if an excessive load is applied to the neck during low-loop bonding, the surface layer is sufficiently plastic. It cannot be deformed and the above effect cannot be obtained. On the other hand, when the Meyer hardness of the surface of the bonding wire is less than 0.2 GPa, since the hardness is too small, the surface of the bonding wire is easily damaged in the process of handling the bonding wire, and many surface scratches occur depending on the handling method. There is a case. Here, Meyer's hardness is the hardness measured using a steel ball or cemented carbide ball indenter. The load when the test surface is indented with the indenter is the projected area of the diameter of the permanent indentation. The value has a dimension of stress. Since the Mayer hardness at a depth of about 1 nm can be measured using a material surface analysis method called the nanoindentation method, it is preferable to use the nanoindentation method for confirming the Meyer hardness value of the present invention. . The Mayer hardness of the surface of the bonding wire is obtained by measuring the outermost surface of the bonding wire having the alloy layer and the coating layer by the nanoindentation method. A Meyer hardness of 0.2 to 2.0 GPa corresponds to a Vickers hardness of about 50 to 570 Hv.

  Regarding the kind and composition of the additive element in the core wire, the wire drawing according to the present invention is made of copper or a copper alloy, but various additive elements are added to the core wire as long as the effects of the present invention are not impaired. It may be added. Examples of elements that can be added to the core wire include Ca, B, P, Al, Ag, Se, and the like. Among these additive elements, it is more preferable to include at least one of B, P, and Se. When the additive elements are contained in a total amount of 5 to 300 ppm by mass, the strength of the bonding wire is further improved. As a result, for example, even when a long loop having a loop length exceeding 5 mm is bonded, the straightness of the loop can be ensured. This is presumably because the additive element contributes to solid solution strengthening or grain boundary strengthening in the copper crystal grains in the core wire. However, if the addition is less than 5 ppm by mass, the effect of further improving the above strength cannot be obtained. On the other hand, the addition exceeding 300 mass ppm may undesirably increase the risk of damaging the chip at the time of ball bonding because the ball portion is excessively cured. For the method of analyzing the component content in the core wire, it is effective to cut the bonding wire and analyze it while digging in the depth direction from the cross-sectional portion by sputtering or the like, and line analysis or point analysis in the cross-section. . In the former method of measuring while digging, if the measurement depth increases, it takes too much measurement time. The latter line analysis or point analysis is advantageous in that it is relatively easy to confirm the concentration distribution over the entire cross section, reproducibility at several locations, and the like. Line analysis is relatively simple in the cross section of the bonding wire, but if you want to improve the accuracy of the analysis, narrow the analysis interval in the line analysis or expand the area to be analyzed in detail, and then perform point analysis. It is also effective to perform. EPMA, EDX, AES, TEM, etc. can be used as an analyzer used for these analyses. In addition, in order to investigate the average composition, it is also possible to use a technique in which the bonding wire is dissolved stepwise from the surface with a chemical solution such as acid, and the composition of the dissolved portion is determined from the concentration contained in the solution. . If the thickness and composition obtained by any one of the above methods are within the scope of the present invention, the effects of the present invention can be obtained.

  The preferred examples of the present invention have been described above, but the present invention can be modified as appropriate. For example, a diffusion layer may be formed between the core wire and the coating layer. For example, the palladium-containing region is a diffusion layer containing less than 50% palladium by diffusing the palladium and copper constituting the core wire continuously with the coating layer. Due to the presence of such a diffusion layer, the bonding wire can improve the adhesion between the coating layer and the core wire.

  Hereinafter, an example is demonstrated about the manufacturing method of the bonding wire of this invention.

  In order to manufacture a bonding wire having the above composition, high-purity copper (purity of 99.99% or more), or after weighing these high-purity copper and additive element materials as starting materials, these are weighed under high vacuum or nitrogen or An ingot of copper or copper alloy is obtained by melting under heating in an inert atmosphere such as Ar. The ingot is finally drawn using a metal die to the required core wire diameter. The coating layer containing palladium according to the present invention is applied after drawing to the final core diameter. As a method for forming the coating layer containing palladium, electrolytic plating, electroless plating, vapor deposition, or the like can be used. However, it is industrially most preferable to use electrolytic plating that can stably control the film thickness. Thereafter, an alloy composed of silver and palladium is formed on the surface of the coating layer. The method may be any method, but after the coating layer is formed, a silver film is further formed as a skin layer on the surface, and the wire is kept at a constant speed in an electric furnace at a constant furnace temperature. The method of accelerating alloying by sweeping continuously is preferable because the composition and thickness of the alloy can be reliably controlled. As a method for further forming a silver film on the surface of the coating layer, electrolytic plating, electroless plating, vapor deposition, or the like can be used. However, it is industrially most preferable to use electrolytic plating for the above reasons. At the time of heating for alloying, considering that silver is easily sulfided, the atmosphere in the furnace is made an inert atmosphere such as nitrogen or Ar. Furthermore, unlike conventional heating methods for bonding wires, The sulfur concentration contained in the atmosphere is set to 900 ppm or less. More preferably, at least 100 ppm of a reducing gas such as hydrogen is mixed in the inert gas, so that the effect of preventing sulfidation of the wire can be further enhanced. Most preferably, in order to avoid as much as possible impurity gas such as sulfur from being introduced from outside the apparatus, an additional second atmosphere furnace is installed outside the atmosphere furnace (first atmosphere furnace). Even if a small amount of impurity gas is mixed in from the outside into the second atmosphere path, these impurity gases may not easily reach the first atmosphere furnace. Moreover, although the suitable temperature in a furnace changes also with the composition of a wire, and the speed which sweeps a wire, when it is set as the range of about 230 degreeC-720 degreeC, it is good because a stable quality bonding wire is obtained. The speed at which the wire is swept during the wire drawing step is preferably about 40 to 80 m / min because stable operation can be performed.

  In the manufacturing method of the bonding wire according to the present invention, the manufacturing method in which the area of the crystal grain having an inclination of <100> crystal orientation with respect to the drawing direction is 15 degrees or less is 50% or more and 100% or less is manufactured by a normal manufacturing method. It is difficult to do and is manufactured in a special way.

  Specifically, after an ingot is obtained as described above, a coating layer containing palladium is formed on the ingot in the same manner as described above. Further, a silver film is formed thereon in the same manner as described above. When the ingot formed with the coating layer and the silver film is drawn to a final core wire diameter using a metal die, the area reduction rate of the die is 11 to 19 at a wire diameter of 80 μm or more. When the wire is drawn at a wire diameter of less than 80 μm, the wire is drawn with a surface reduction rate of about 7 to 17%, which is larger than usual. Thereby, it is possible to develop a texture having a directivity on the silver film (a texture having crystal orientations aligned in the wire drawing direction). However, if the wire is drawn with a large area reduction rate, the risk of breakage increases. Therefore, in order to prevent the bonding wire from breaking, the wire drawing speed should be lower than usual, for example, 4 to 8 m / min. Is more preferable. Also in this bonding wire, after wire drawing, heat treatment for promoting alloying is performed in the same manner as described above. If the temperature in the heat treatment process that promotes alloying after wire drawing is low, the ratio of the area of the crystal grains with the inclination of the <100> crystal orientation with respect to the wire drawing direction is 15 degrees or less is increased. The area ratio decreases. This reduction in area is caused by the tendency to lose the directionality in the texture described above when recrystallization is promoted by heating in this step. Specifically, if the furnace temperature is 230 ° C. to 280 ° C., the area ratio is 100%, and if the furnace temperature is in the range of 680 ° C. to 720 ° C., the area ratio is 50%. The area ratio can be controlled by the temperature of the heat treatment.

  In the manufacturing method of the bonding wire of the present invention, the manufacturing method of the bonding wire in which the Mayer hardness of the surface of the coating layer is in the range of 0.2 to 2.0 GPa is difficult to manufacture by a normal manufacturing method, An alloy of silver and palladium on the surface is manufactured to be exceptionally soft. Specifically, the wire was drawn to the target wire diameter by any of the methods described above, and after the heat treatment step for alloying was completed, the bonding wire was further controlled in an argon atmosphere for each spool. It can be manufactured by installing in an electric furnace and heating at 150 to 200 ° C. for 20 to 24 hours. By heating at a temperature lower than 150 ° C. or shorter than 20 hours, the alloy of silver and palladium cannot be made particularly soft as the above-mentioned hardness. When heating at a temperature higher than 200 ° C. or longer than 24 hours, diffusion between adjacent wires is promoted, and the wires may stick to each other.

  Examples will be described below.

  As a raw material for the bonding wire, copper used for the core wire, B, P, Se, Ca, Al as additive elements in the core wire, palladium used for the coating layer, silver used for the skin layer has a purity of 99.99% by mass or more. Each material was prepared. After weighing the above-mentioned copper or copper and additive element material as a starting material, this was heated and melted under high vacuum to obtain an ingot having a diameter of about 10 mm of copper or copper alloy. Thereafter, forging, rolling, and wire drawing were performed to produce a wire having a predetermined diameter. Thereafter, a coating layer containing palladium was formed on the surface of each wire by electrolytic plating. Here, the thickness of the coating layer was controlled by the time of electrolytic plating. Thereafter, a silver film is formed by electroplating on the surface of the coating layer, and the wire is continuously swept at a speed of 60 m / min in a furnace maintained at 300 to 800 ° C. An alloy layer of silver and palladium was formed on the surface. Here, the thickness of the alloy layer was controlled by the basis weight of the silver film, that is, the electroplating time. In this way, a bonding wire having a diameter of 20 μm was obtained. In some samples, in order to control the area of the crystal grains whose inclination with respect to the drawing direction of the <100> crystal orientation is 15 degrees or less, the area reduction rate of the die is set to a thickness of 80 μm or more. Drawing was performed at about 13 to 18%, and at the time of drawing at a thickness of less than 80 μm, the above-mentioned area reduction was about 8 to 12%, which was drawn with a larger area reduction than usual. Moreover, in some samples, in order to control the Mayer hardness of the surface of the coating layer, the bonding wire and the spool are placed in an electric furnace controlled in an argon atmosphere, and the temperature is 150 to 200 ° C. for 20 to 24 hours. Heat was applied.

  The diameter of the core wire, the thickness of the coating layer and the alloy layer in the resulting bonding wire are analyzed by AES while sputtering the surface of the bonding wire, and the bonding wire is cross-polished and measured while analyzing the composition by EDX. did. A region in which the concentration of palladium is 50% or more and the concentration of silver is less than 10% is used as a coating layer, and the silver concentration in the alloy layer containing silver and palladium on the surface of the coating layer is 10 to 70%. The region that was in the range was used as the alloy layer. The thickness and composition of the coating layer and the alloy layer are shown in Tables 1 to 5, respectively.

  To evaluate the anti-oxidation effect of the bonding wire by the coating layer, leave the bonding wire together with the spool for 72 hours in a high-temperature and high-humidity furnace with a humidity of 85% and a temperature of 85 ° C to promote oxidation of the wire surface. Accelerated tests were conducted. After heating, the bonding wire was taken out from the high-temperature and high-humidity furnace, and the degree of surface oxidation was observed with an optical microscope. At this time, if the entire surface of the wire was oxidized, it was marked with “x”, and if the wire surface was not oxidized, it was marked with “◯” in the column of “long-term storage” in Tables 1 and 5.

  A commercially available automatic wire bonder was used to connect the bonding wires. A ball portion was produced at the tip of the bonding wire by arc discharge immediately before bonding, and its diameter was set to 34 μm so as to be 1.7 times the diameter of the bonding wire. Nitrogen was used as an atmosphere for producing the ball part.

  The actual diameter of the ball part is measured with 20 SEMs for each ball part, and if the difference between the maximum value and the minimum value exceeds 10% of the average value of the ball diameter, the variation is severe and poor. If x is more than 5% and less than 10%, △ is intermediate, and if more than 3% and less than 5%, there is no practical problem and ○ is extremely less than 3%. “Good” was marked in the column of “FAB sphericity in nitrogen” in Tables 1 and 5.

  Further, when the ball portion was observed with an SEM and bubbles were observed in its appearance, this was described in the column of “Inhibition of FAB bubbles in nitrogen” in Tables 1 and 5. Furthermore, 10 ball sections are polished by a cross section and observed with an optical microscope. If no bubbles are observed in the cross section, it is very good, and ◎ marks indicate that bubbles are present only in 1 to 2 of the 10 ball sections. If it is observed, it is marked as ◎, and if bubbles are observed only in 3 to 4 ball parts out of 10, if there is no problem in practical use, it is marked as ◯ and bubbles are found in 5 or more of 10 ball parts. If it is observed, it was marked as “bad” by “×” in the column of “Inhibition of FAB bubbles in nitrogen” in Tables 1 and 5.

  As bonding partners of the bonding wires, an Al electrode having a thickness of 1 μm formed on a Si chip and a lead having a surface of silver or palladium plated lead frame were used. After the ball portion thus prepared is bonded to the electrode heated to 260 ° C., the bus wire portion of the bonding wire is wedge-bonded to the lead heated to 260 ° C., and the ball portion is formed again, thereby continuously bonding. Repeated. The loop length was set to 4.9mm. In some samples, the above-mentioned reverse bonding with a loop length of about 1 mm is used, and for other samples, a low loop bonding with a loop length of about 3 mm and a loop height of 76.2 μm (3 mil) is used. Each sample was subjected to long bonding with a loop length of 5.3 mm (210 mil).

  Regarding the wedge bondability of the bonding wire, the wedge-bonded bonding wire is picked immediately above the wedge joint, lifted upward until it is cut, and the breaking load obtained at the time of cutting is read, so-called peel strength measurement method. The breaking load (peel strength) of the book was measured. If the standard deviation of the peel strength exceeds 5mN, the variation is large and needs to be improved. If it is less than 5mN, there is no practical problem. “2nd bonding” (for silver-plated lead frame leads) and “Pd-L / F 2nd bonding” (for palladium-plated lead frame leads) are shown in the columns.

  Here, it was observed with an optical microscope whether or not the loop was damaged by the capillary. The number of loops observed was 20 and it was very good if there was no flaw, and ◎◎ marked, and when only one or two loops were scratched, it was good and ◎ marked 3-4 When scratches are observed only on the loops of the book, ○ marks at a level where there is no practical problem. Poor marks if scratches are observed on five or more loops. It was written in the column.

  The inclination of the <100> crystal orientation of the crystal grains observed on the surface of the coating layer with respect to the drawing direction was calculated after observing the orientation of the individual crystal grains by the EBSD method. In the calculation, a surface having a width of 8 μm in the direction perpendicular to the drawing direction of the bonding wire and a length of 150 μm in the drawing direction of the bonding wire was observed for each sample in three fields. The value was described in the column of “Area of crystal grain having inclination of <100> crystal orientation with respect to drawing direction of 15 ° or less” in Tables 2-4.

  The wrinkles on the bonding wire surface after the reverse bonding described above are very good if there are no wrinkles in each sample when 20 loops are observed with an optical microscope. If only wrinkles are observed in the loop, it is good and marked with ◎, and if wrinkles are observed only in 3 to 4 loops, it is marked with ○ mark at a level where there is no practical problem, and wrinkles are observed in 5 or more loops. If it is inferior, it is indicated by a “x” mark in the “Reverse wrinkle suppression” column of Tables 2-4.

  The Mayer hardness of the wire surface was measured with a depth accuracy of about 1 nm by the nanoindentation method, and the value was listed in the “Meyer hardness of the wire surface” column of Tables 3-4.

  The presence or absence of damage in the neck portion after the low-loop bonding described above was observed if 20 loops of each sample were observed with an optical microscope. If damage is observed with 2 pieces, it is marked with a circle with no problem level, and if damage is observed with 3 or more pieces out of 20 pieces, it is inferior and marked with x mark, “76.2 μm (3mil) class low in Table 3-4” It is described in the “Loop Neck Damage” column.

  Regarding the bending of the loop after the long bonding, 20 loops of each sample were measured using a projector. Here, the value obtained by dividing the average value by the loop length is the wire bending rate, and if it is less than 4%, it is very good, and if it is 4-5%, it is marked as ◯ as a practically no problem level, If it exceeds 5%, it is judged as defective, and is indicated in the column of “5.3 mm (210 mil) long bend” in Table 4 with a cross.

  As described in Examples 1 to 36 of Table 1, a palladium coating layer having a thickness of 10 to 200 nm is formed on the surface of the copper core wire, and silver and palladium having a thickness of 3 to 30 nm are further formed on the surface of the coating layer. In the bonding wire having the alloy layer, the oxidation resistance ("long-term storage" column) and the true sphericity of the ball part ("FAB true sphericity in nitrogen" column) are secured, and on the palladium plated lead frame. Good wedge bondability ("Pd-L / F 2nd bond" column) can be obtained. On the other hand, as shown in Comparative Example 1, long-term storage and 2nd bondability are inferior only with a core wire that is not particularly provided with a coating layer on a copper wire. Moreover, as shown in Comparative Example 2, when the coating layer on the surface of the copper core wire is silver, the sphericity of the ball portion in nitrogen is inferior. Moreover, as shown in Comparative Examples 3 to 5, when only the palladium coating layer is provided on the copper core wire, the wedge bondability on the palladium plated lead frame is poor. Moreover, as shown in Comparative Example 6, even when a palladium coating layer is formed on a copper core wire with a thickness in the range of 10 to 200 nm, the thickness of the alloy layer of silver and palladium formed on the surface of the palladium coating layer is further increased. Is thinner than 3 nm, the wedge bondability on the palladium-plated lead frame is poor. Moreover, as shown in Comparative Example 7, even when a palladium coating layer is formed on a copper core wire with a thickness in the range of 10 to 200 nm, the thickness of the alloy layer of silver and palladium formed on the surface thereof is further increased. When the thickness is larger than 30 nm, it is difficult to ensure stable quality, and any of the evaluated characteristics is inferior because the alloy layer is oxidized or sulfided. Further, as shown in Comparative Example 8, a palladium coating layer was formed on the copper core wire with a thickness in the range of 10 to 200 nm, and the silver concentration in the alloy layer of silver and palladium formed on the surface was further increased. Is less than 10%, the wedge bondability on the palladium plated lead frame is poor. Further, as shown in Comparative Example 9, a silver coating concentration was formed on the copper core wire with a thickness in the range of 10 to 200 nm, and the silver concentration in the alloy layer of silver and palladium formed on the surface thereof was further increased. Is higher than 70%, the sphericity of the ball part in nitrogen is inferior. Moreover, as shown in Comparative Example 10, when the thickness of the palladium coating layer formed on the copper core wire exceeds the range of 10 to 200 nm, the thickness of the alloy layer of silver and palladium formed on the surface is further increased. Even in the range of 3 to 30 nm, bubbles are generated when a small-diameter ball portion is formed in nitrogen ("FAB bubble suppression in nitrogen" column).

  As shown in Examples 10 to 36, when the silver concentration in the alloy composed of silver and palladium is 20% or more, the effect of suppressing the occurrence of flaws by the capillaries is larger (“Scratch Control” column), and Even if a small-diameter ball portion is formed in nitrogen, the generation of bubbles is suppressed ("FAB bubble suppression in nitrogen" column). Furthermore, as shown in Examples 19 to 36, when the silver concentration was 30% or more, the above-described effect was further enhanced.

  As shown in Examples 37 to 52 in Table 2, the area of the crystal grains whose inclination with respect to the drawing direction of the <100> crystal orientation observed on the surface of the bonding wire is 15 degrees or less is 50% or more and 100% or less When it was, the effect of suppressing wrinkles generated on the surface of the loop during reverse bonding was increased (“Reverse Wrinkle Control” column), and the effect was further increased when the area was 70% or more.

  As shown in Examples 53 to 56 and 59 to 62 in Table 3, when the Mayer hardness of the surface of the bonding wire is in the range of 0.2 to 2.0 GPa, neck damage is suppressed even when low loop bonding is performed. ("76.2 μm (3 mil) class low loop neck damage" column).

  As shown in Examples 68 to 76 and 80 to 83 in Table 4, when the core wire contains a total of 5 to 300 ppm by mass of at least one of B, P, and Se, long bonding is performed. Even when it is performed, the bending of the loop is suppressed ("5.3 mm (210 mil) class long bending" column).

  As shown in Examples 84 to 93 in Table 5, even if a diffusion layer is formed between the coating layer and the core wire, or copper contained in the core wire is diffused in the coating layer, The effect of the present invention can be secured.

Claims (5)

A core wire made of copper or copper alloy;
A coating layer containing palladium formed on the surface of the core wire with a thickness of 10 to 200 nm;
On the surface of the coating layer, an alloy layer of silver and palladium formed with a thickness of 3 to 30 nm,
A bonding wire for a semiconductor, wherein a concentration of silver in the alloy layer is 10% by volume or more and 70% by volume or less. 2. The area of crystal grains in which the inclination of the <100> crystal orientation with respect to the wire drawing direction is 15 degrees or less among the surface crystal grains of the alloy layer is 50% or more and 100% or less. Bonding wire for semiconductor. 3. The bonding wire for a semiconductor according to claim 1, wherein a Mayer hardness of the surface of the bonding wire is in a range of 0.2 to 2.0 GPa. 4. The bonding wire for a semiconductor according to claim 1, wherein a concentration of silver in the alloy layer is 20 vol% or more and 70 vol% or less. 5. The bonding wire for a semiconductor according to claim 1, wherein the core wire contains at least one of B, P, and Se in a total amount of 5 to 300 ppm by mass.