WO2013094482A1 - Pd被覆銅ボールボンディングワイヤ - Google Patents

Pd被覆銅ボールボンディングワイヤ Download PDF

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Publication number
WO2013094482A1
WO2013094482A1 PCT/JP2012/082137 JP2012082137W WO2013094482A1 WO 2013094482 A1 WO2013094482 A1 WO 2013094482A1 JP 2012082137 W JP2012082137 W JP 2012082137W WO 2013094482 A1 WO2013094482 A1 WO 2013094482A1
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WIPO (PCT)
Prior art keywords
palladium
gold
copper
layer
wire
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Application number
PCT/JP2012/082137
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English (en)
French (fr)
Inventor
満生 高田
山下　勉
執行　裕之
岳 桑原
純一 岡崎
茂 斉藤
Original Assignee
田中電子工業株式会社
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Publication date
Application filed by 田中電子工業株式会社 filed Critical 田中電子工業株式会社
Priority to SG2013032347A priority Critical patent/SG191711A1/en
Priority to CN201280007491.1A priority patent/CN103339719B/zh
Publication of WO2013094482A1 publication Critical patent/WO2013094482A1/ja

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Definitions

  • the present invention relates to a Pd-coated copper wire used for connecting an electrode on a semiconductor element and a wiring of a circuit wiring board by ball bonding.
  • gold wires having a wire diameter of about 15 to 30 ⁇ m are mainly used as bonding wires for bonding between electrodes on semiconductor elements and external terminals by ball bonding.
  • the use of copper wires having a wire diameter of about 10 to 25 ⁇ m is drawing attention in place of conventional high-purity 4N-based (purity of 99.99 mass% or more) gold wires.
  • This copper wire is also considered to be used in the same field as gold wire.
  • QFP Quad Flat Packaging
  • BGA Ball
  • CSP Chip Scale Packaging
  • the material with which the copper wire bonding wire is joined is the same as that of the gold wire, and the wiring on the silicon substrate and the electrode material can be made into finer wiring in addition to the conventional aluminum (Al) alloy pad.
  • Suitable high purity copper (Cu) has been put into practical use.
  • silver (Ag) plating, gold (Au) plating, and palladium (Pd) plating on nickel (Ni) plating are applied on the lead frame, and on the resin substrate, tape, etc.
  • a copper (Cu) wiring is provided, and a film of a noble metal element such as gold (Au) and an alloy thereof is provided thereon.
  • Non-patent Document 1 it is known that the growth of aluminum (Al) oxide can be suppressed if palladium (Pd) can be dispersed at the bonding interface between the pad and the wire.
  • the palladium-coated copper wire itself has been conventionally known, but is mainly used for oxidation prevention of a copper core material that is easily oxidized and for wedge bonding to a palladium-coated lead frame. Even when the coating layer was provided, it was not incorporated into the copper of the core material at the time of forming the molten ball, and the necessary concentration of palladium was not dispersed in the essential ball surface layer. Generally, these palladium coating layers are relatively thin because the material palladium is expensive, and the proportion of the expensive palladium (Pd) coating layer to the copper (Cu) core material is 1/20 or less.
  • the palladium-coated copper wire described in Patent Document 4 is a wire for performing wedge bonding with a palladium-coated lead frame, and an alloy containing gold and palladium having a thickness of 3 to 80 nm on the surface of the palladium-coated layer. A layer is formed.
  • palladium is alloyed in this manner, it diffuses into copper as a core material, and a palladium dispersion layer on the surface of the molten ball cannot be formed.
  • the present inventors arrange gold (Au) having a melting point lower than that of copper (Cu) of the core material on the outside of the copper (Cu) of the core material, and induce gold (Au) as water to induce copper (Cu) of the core material.
  • palladium (Pd) is three-dimensionally grown in a gold (Au) coating layer to form a mixed layer in which gold (Au) and palladium (Pd) are mixed.
  • the structure formed by this Stransky Krostanov-type growth is known to occur in an extremely thin metal layer and form an island structure grown from the underlying metal to impair the flatness of the surface layer (Patent Document) 5)
  • the structure formed by the Stranky clusternov growth is such that the gold layer formed on the palladium coating layer is extremely thin, and palladium is 3 in the gold layer having a thickness of several nanometers or less. It is formed by dimensional growth and has a structure in which a fine palladium phase and a gold phase are mixed.
  • Gold (Au) and palladium (Pd) are inherently completely mixed in all proportions, and the crystal lattice is only slightly different by about 4.9%, so the adhesion between the two layers of gold (Au) / palladium (Pd)
  • a palladium (Pd) underlayer can be deposited on the surface of the gold (Au) layer by utilizing the Stranky-Clusternov growth.
  • the mixed layer in which gold (Au) and palladium (Pd) are mixed is unstable, and the higher the purity of gold (Au) and palladium (Pd), the easier it is to form an alloy. Therefore, in order to stabilize the mixed layer in which gold (Au) and palladium (Pd) are turbulent, the present inventors brought hydrogen gas into contact with the surface of palladium (Pd) at a high temperature and hydrogen in palladium (Pd). We decided to make the mixed layer more stable by absorbing atoms. It is more preferable to mix alcohol such as ethanol in the aqueous solution to be rapidly cooled after the heat treatment, because the rapidly cooled palladium (Pd) surface layer takes in hydrogen atoms thermally decomposed by alcohol.
  • alcohol such as ethanol
  • the present inventors appropriately adjust the final heat treatment after continuous wire drawing, and finally adjust the elongation / tensile strength of the heat treatment using the above-mentioned Transky / Clusternov growth and the copper (Cu) bonding wire.
  • a more stable mixed layer structure in which gold (Au) and palladium (Pd) are mixed is surely maintained. Decided to do.
  • the present inventors appropriately adjust the final heat treatment after continuous wire drawing, thereby performing heat treatment using the above-mentioned transky clusternov growth and high-temperature treatment for absorbing hydrogen atoms in palladium (Pd).
  • the present invention aims to solve the above-described problems, and specifically has the following configuration.
  • the coated copper wire for ball bonding of the present invention is a ball bonding having a surface coated wire diameter of 10 to 25 ⁇ m consisting of a core material made of copper (Cu) or a copper alloy and an intermediate coating layer made of palladium (Pd).
  • the intermediate coating layer is palladium (Pd) having a purity of 99% by mass or more, and further, an uppermost layer of gold (Au) is formed on the intermediate layer, and the core material of the intermediate coating layer Is a mixed layer having an average cross-sectional thickness of 5 nm or less observed by a scanning electron microscope in which the palladium (Pd) and gold (Au) having a purity of 99.9% by mass or more are disturbed by thermal growth on the bonding interface on the opposite side. And the palladium surface of the mixed layer is subjected to hydrogen diffusion treatment.
  • the specific aspect of the coated copper wire for ball bonding of the present invention is as follows. (2) The coated copper wire for ball bonding according to (1), wherein the average thickness of the cross section of the mixed layer is 3 nm or less. (3) The coated copper wire for ball bonding according to (1), wherein the average thickness of the cross section of the mixed layer is 1 nm or less. (4) The coated copper wire for ball bonding according to (1), wherein the palladium (Pd) is wet-plated. (5) A copper wire coated with gold (Au) and palladium (Pd) in which the mixed layer has been subjected to strong wire drawing is subjected to the transfer in the gold (Au) coating layer by transkey clusteroff growth.
  • the coated copper wire for ball bonding according to (1) above wherein (Pd) is three-dimensionally grown to form a mixed layer in which gold (Au) and palladium (Pd) are mixed.
  • the above (1) was performed in a hydrogen-containing inert atmosphere at 450 ° C. to 700 ° C. with respect to a copper wire coated with gold (Au) and palladium (Pd) in which the mixed layer was subjected to strong wire drawing.
  • the ball according to (1) comprising 0.5 to 99 mass ppm of phosphorus and 0.1 to 500 mass ppm of phosphorus (P), and the balance being copper (Cu) having a purity of 99.9 mass% or more Coated copper wire for bonding.
  • the present invention at the time of forming a molten ball of the first bond, first, gold (Au), which is easily melted, is used as an invitation water to melt copper (Cu) as a core material, and then palladium (Pd) in contact with copper (Cu).
  • Au gold
  • Cu copper
  • Pd palladium
  • the palladium (Pd) on the surface that occluded hydrogen atoms was melted at the end, so that the palladium (Pd) was uniformly and finely deposited on the surface of the molten ball in contact with the aluminum (Al) interface on the pad.
  • Au gold
  • the present invention appropriately adjusts the film thickness of gold (Au) having a purity of 99.9% by mass or more and the final heat treatment, so that it is not an alloy layer of gold (Au) and palladium (Pd) but gold (Au). ) And palladium (Pd) were mixed and a mixed layer could be formed. Furthermore, by introducing hydrogen gas during the final heat treatment, the palladium (Pd) exposed on the gold (Au) surface layer reacts with hydrogen molecules, and the hydrogen atoms are absorbed into the palladium (Pd). It was possible to stabilize the mixed layer in which (Au) and palladium (Pd) were mixed up more reliably.
  • the wire structure of the present invention when the molten ball of the first bond is formed, first, gold (A) having the lowest melting point (1064 ° C.) in the outermost surface layer of the Stransky-Clusternov structure starts to melt and is mixed in the same layer.
  • the surface of the copper core material that flows along with the palladium to be exposed and reaches the wire end surface from the palladium layer of the intermediate layer is covered.
  • Copper (Cu) has a melting point higher than that of gold (Au) and palladium (Pd) (1085 ° C.), but immediately melts in direct contact with the gold (Au) melted at the end surface of the wire. ) To quickly melt as water to form a ball.
  • Palladium (Pd) which has the highest melting point in the structure of the wire (1555 ° C.), once remains in the form of a thin sheath while the copper melts first, but melts while the molten ball of copper is formed.
  • the palladium in the mixed layer that reaches the end surface of the wire with the previously melted gold (Au) is stabilized by the occluded hydrogen atoms and is not alloyed. It is believed that it melts in the vicinity of the ball surface and segregates in the vicinity of the ball surface to maintain a high palladium (Pd) concentration.
  • the bonding wire of the present invention has the above-described structure, aluminum oxide or the like is not generated at the bonding interface between the wire and the pad even when left at high temperature for a long time, and is excellent in long-term high-temperature stability. ing.
  • the upper limit of the theoretical thickness of the ultra-thin surface layer is 10 nm, and the upper limit is preferably 8 nm, and more preferably 7 nm or less.
  • the “theoretical thickness” means that it is very difficult to directly measure such an ultra-thin total surface layer. Therefore, the “theoretical thickness” is based on the thickness of the surface layer during dry plating before continuous drawing. It is obtained by proportional calculation.
  • the proportionality constant for obtaining “theoretical thickness” is a value obtained by dividing the wire diameter after the end of continuous drawing as a bonding wire by the diameter of the wire before the start of continuous drawing.
  • the actual thickness can be directly observed with a high-magnification scanning electron microscope, it can be obtained only with an approximate average film thickness because it is nano-order even when viewed with a theoretical thickness.
  • Au gold
  • Pd palladium
  • the interaction between gold (Au) and the interaction between gold (Au) and palladium (Pd) is stronger, and palladium (Pd) can be exposed from the gold (Au) surface.
  • the hydrogen atoms cannot be absorbed into palladium (Pd).
  • the ultra-thin gold (Au) surface layer is preferably coated by sputtering from the viewpoints of throwing power and bondability with the intermediate coating layer.
  • Gold (Au) or the like coated by the sputtering method becomes hard due to collision of gas molecules even with a purity of 99.9% by mass or more, and has good throwing power.
  • Surface layer such as gold (Au) penetrates into the fine uneven surface of the palladium (Pd) precipitate which becomes relatively harder during continuous cold drawing, and joins firmly during cold drawing Is done.
  • the ultra-thin surface layer such as gold (Au) coated by the sputtering method does not form a geometrically uniform circular film in the circumferential cross-sectional shape of the wire, but at the time of forming the molten ball It is rapidly absorbed into the core material copper (Cu) by the surface tension, the non-uniformity of the surface layer is eliminated, and a true spherical molten ball is formed.
  • Au gold
  • the core material contains 1 to 80 ppm by mass of phosphorus (P), and the balance is made of copper (Cu) having a purity of 99.999% by mass or more.
  • P phosphorus
  • Cu copper
  • purity of 99.999% by mass or more means that impurity elements of metals other than phosphorus (P) and copper (Cu) are less than 0.001% by mass, and exist in copper (Cu).
  • gaseous elements such as oxygen, nitrogen and carbon.
  • the copper (Cu) ball deoxidizes in the process of solidifying the molten copper ball in the first bonding. If the purity of copper (Cu) is 99.999% by mass or more, phosphorus (P) deoxidizes in the range of 1 ppm by mass or more, and if it is in the range of 80 ppm by mass or less, copper (Cu) is drawn at the time of wire drawing.
  • the core material does not work harden.
  • the intermediate layer is made of palladium (Pd).
  • the melting point (1554 ° C.) of palladium (Pd) is higher than either the melting point of copper (Cu) (about 1085 ° C.) or the melting point of a copper alloy to which a trace element is added. Therefore, in the first stage in which the core material copper (Cu) or copper alloy forms a spherical molten ball, palladium (Pd) becomes a thin skin to prevent oxidation from the side surface of the molten ball.
  • the purity of the palladium intermediate layer does not affect the sphericity of the molten ball so much, and even if it is thin or thick, it does not become eccentric, but it requires palladium (Pd) with a purity of 99% by mass or more for continuous drawing. It is. Although the thickness of the intermediate layer is appropriately determined, the thicker the palladium (Pd) intermediate layer, the slower the deterioration of the copper (Cu) or copper alloy core material.
  • dry plating or wet plating can be used as a method for forming the palladium (Pd) intermediate coating layer.
  • dry plating sputtering, ion plating, vacuum deposition, or the like can be used. From the viewpoint of avoiding contamination of impurities, dry plating is preferable, but wet plating is preferable for obtaining an annular shape with a uniform cross section.
  • an electrolytic plating bath of palladium (Pd) is preferably an ammoniacal aqueous solution or a cyanic aqueous solution free of halogen ions or sulfate ions in order to maintain the purity.
  • the polymer compound or the metal salt brightener adversely affects the true sphericity of the molten ball, it is preferably not included as a film component. Since the film electroplated on a core material such as copper (Cu) is strongly compressed by subsequent continuous wire drawing, the film properties are not as important as the film components.
  • Palladium (Pd) electroplating baths include palladium p-salt (Pd (NH3) 2 (NO2) 2), ammonium nitrite and potassium nitrate, or palladium p-salt (Pd (NH3) 2 (NO2) 2), ammonium nitrate and A weak alkaline aqueous ammonia solution of ammonia water, a neutral ammonia aqueous solution of Pd (NH 3) 2 (COO) 2 and (NH 4) 2 HPO 4 (US Pat. No.
  • the particle size of the precipitate tended to increase as the pH increased.
  • the purity of copper (Cu) is preferably 99.999% by mass or more rather than 99.99% by mass in order to prevent abnormal deposition of a sputtered film or the like.
  • palladium (Pd), platinum (Pt), nickel, Strike plating (ultra-thin plating) such as (Ni) can also be performed.
  • the melting point of the metal in each coating layer is very important. Since most of the bonding wires are occupied by a high purity copper (Cu) core material, the melting point of copper (Cu) (about 1085 ° C.) is used as a standard. It is known that high-purity copper (Cu) as a core material becomes a perfect spherical shape by arc discharge in a reducing atmosphere. It is also known that a high-purity copper (Cu) core material coated with palladium (Pd) becomes a true sphere by arc discharge in a non-oxidizing atmosphere.
  • the copper (Cu) Since the melting point (about 1555 ° C.) of palladium (Pd) is higher than the melting point (about 1085 ° C.) of copper (Cu), the copper (Cu) is dragged into a true spherical shape in a non-oxidizing atmosphere. It is considered to be spherical.
  • high-purity gold (Au) bonding wire can be used to form high-purity gold (Au) with high-purity copper (Au) regardless of the atmosphere.
  • the bonding wire directly coated on the core material of Cu) has a bowl shape, and a spherical ball cannot be obtained.
  • the melting point of gold (Au) (about 1064 ° C.) is lower than the melting point of copper (Cu) (about 1085 ° C.)
  • the low melting point gold is formed at the stage where copper (Cu) forms a spherical molten ball.
  • the (Au) surface layer melts sooner than copper (Cu) and tries to wrap the wire end face quickly, but high melting point palladium (Pd) is in the way.
  • gold (Au) having a low melting point is preferentially diffused into copper (Cu), and is absorbed by molten copper (Cu), and then palladium (Pd) is considered to melt.
  • Trace addition elements in copper (Cu) have little effect on the melting phenomenon.
  • the thickness of the outermost gold (Au) layer affects the sphericity of the molten ball.
  • the gold ( Au) promotes the melting of copper (Cu) at the end face of the wire, but the gold (Au-palladium (Pd) mixed layer in the present invention is on the order of several nm, and the amount of gold (Au) is very small. The effect is suppressed and the sphericity of the molten ball is not adversely affected.
  • the wet plating of the palladium (Pd) intermediate layer does not include a leveling agent or a brightening agent, it tends to be deposited in an irregular granular shape. Further, the dry plating tends to be deposited in layers along the crystal plane of the high purity copper (Cu) core material. In either case, the wire cross section is not a perfect ring shape, but it is sufficient if the thickness of the surface layer is 1 nm or more.
  • a surface layer of high-purity gold (Au) or the like can be moved not only by spattering but also by rotating the wire around the axis during sputtering, by reciprocating the wire during sputtering, High purity gold (Au) can be deposited on the intermediate coating layer in a more uniform thickness by sputtering from both sides. Since the surface layer such as high-purity gold (Au) has good ductility, it can be continuously drawn to the final wire diameter according to the die hole shape of the diamond die.
  • the gap at the interface between the surface layer of gold (Au) or the like and the intermediate coating layer is filled, and even if there is an abnormal precipitation or the like in the wet plating of the palladium (Pd) intermediate layer,
  • the surface of the bonding wire is entirely covered with gold- (Au) or the like without breaking through and depositing.
  • the continuous wire drawing is preferably a wet wire drawing performed in a coolant. Since the outermost coating layer is thin, the ultra-thin surface layer gold (Au) may diffuse into the copper (Cu) core material and disappear due to the heat accompanying strong compression in dry wire drawing. Because.
  • a metal lubricant with a commercially available surfactant is diluted with a diluent such as water or alcohol. Ethyl alcohol, methyl alcohol or isopropyl alcohol It is preferable that the wire is continuously drawn in a solution such as an aqueous solution containing only the.
  • a copper wire drawn from a copper (Cu) ingot shown in Table 1 to a wire diameter of 500 ⁇ m is used as a core material, and 2.0 ⁇ m of electrolytic plating of a palladium (Pd) intermediate layer is deposited on the wire surface by a normal method. It was.
  • a palladium (Pd) plating bath a neutral dinitrodiammine palladium bath to which 10 gW / l phosphate was added was used, and the purity of the obtained palladium (Pd) was 99%.
  • gold (Au) having a purity of 99.99% by mass was magnetron sputtered at room temperature to deposit 0.08 ⁇ m.
  • the plating thickness was measured by Auger electron spectroscopy (AES). Thereafter, this coated copper wire was die-drawn to a final diameter of 17 ⁇ m.
  • the theoretical film thickness of gold (Au) is 0.0027 ⁇ m.
  • a predetermined final heat treatment was performed so that the processing strain was removed and the elongation value was about 10%.
  • a heat treatment furnace having a length of 700 ° C. in a 5% hydrogen + nitrogen atmosphere was passed through a length of 50 cm at 8 m / second and cooled in a 10% ethanol aqueous solution (20 ° C.).
  • Example 1 The coated copper wire produced under the same conditions as in Example 1 was passed through a heat treatment furnace at 600 ° C. in a 5% hydrogen + nitrogen atmosphere at 5 m / sec and cooled in pure water (40 ° C.).
  • the final heat treatment conditions were the same as in Example 2 except that a heat treatment furnace at 800 ° C. was passed at 8 m / second in a 5% hydrogen + nitrogen atmosphere.
  • Table 2 shows the manufacturing conditions for the wires of the above examples and comparative examples.
  • a commercially available automatic wire bonder (ultrasonic thermocompression wire bonder “MAX ⁇ m Ultra (trade name)” manufactured by K & S Co., Ltd.) was used for ball / stitch bonding.
  • the molten ball was produced at the tip of the wire by arc discharge in a gas atmosphere using a Max ⁇ m plus Copper Kit using a mixed gas consisting of 4% by volume hydrogen and the balance nitrogen at a flow rate of 0.5 (l / min). It is bonded to a 0.8 ⁇ m aluminum (Al-0.5% Cu) electrode film on a silicon substrate, and the other end of the wire is plated with 4 ⁇ m silver (Ag) at 200 °C (material is 42 alloy, film thickness is 150 ⁇ m) Stitched on top.
  • the capillaries manufactured by SPT were used, and the wire bonder setting value for the molten ball was adjusted so that the EFO Fire Mode was Bal Size and the FAB Size was twice the actual molten ball diameter.
  • the bonding conditions during bonding were adjusted so that the crimp diameter was 2.5 times the wire diameter.
  • the electrical resistance was measured with a dedicated IC socket and a dedicated automatic measurement system using a product name “source meter (model 2004)” manufactured by KEITHLEY.
  • the measuring method is the so-called DC four-terminal method.
  • a constant current is passed from the measurement probe to the adjacent external leads (a pair in which the pads on the IC chip are short-circuited), and the voltage between the probes is measured.
  • 100 pairs of external leads (200 pins) were measured before and after leaving, and those having an increase rate of 20% or more were determined as defective.
  • a sample having a long time until the defect rate of each sample reached 50% was judged good. If the time is 200 hours or more, it is judged that there is no serious problem in practical use. ⁇ , if it is 150 hours or more and less than 200 hours, ⁇ , if it is 100 hours or more to less than 150 hours, ⁇ , 100 hours When it was less than, it was marked with a cross.
  • Bond strength evaluation was conducted using a lead frame at 200 ° C. with 4 ⁇ m silver (Ag) plating on both ends of the wire without using the above aluminum (Al-0.5% Cu) electrode film (material is 42 alloy, Ball / stitch bonding was performed on the film thickness of 150 ⁇ m. 3920 wires were bonded, the number of non-bonding times was 0-1, ⁇ , 2-3, ⁇ , 4-20, ⁇ , 21 or more ⁇ . These results are shown in Table 4.
  • On-axis eccentricity evaluation is performed using an ultrasonic thermocompression wire bonder “MAX ⁇ m Ultra (trade name)” manufactured by K & S. FAB was continuously created with the parameter set to FAB Mode and evaluated. Bonding was performed continuously on a lead frame of 200 ° C. plated with silver (Ag) with a thickness of 4 ⁇ m, and an aluminum (Al-0.5% Cu) electrode film was not used. The other set values for wire bonding were performed in the same manner as the damage evaluation of the aluminum (Al-0.5% Cu) electrode film. The determination was made by observing 200 molten ball shapes before joining and determining whether the on-axis eccentricity and dimensional accuracy were good.
  • Example 1 and Example 2 are different in heat treatment conditions and cooling conditions as shown in Table 2, and there is almost no difference in the thickness of the mixed layer, but the evaluation is slightly reduced. To do.
  • Comparative Example 1 has no difference in the thickness of the mixed layer, but has not obtained good results because hydrogenation is not performed in the heat treatment atmosphere.
  • the Au coating method is Au plating, the adhesion and density of the film is lower than that of wet plating, there is a locally uneven film thickness, and impurities such as additives and pH adjusting materials As a result, a good result cannot be obtained.
  • the thickness of a mixed layer greatly exceeds the range of this invention, and the effect is not acquired. That is, it can be seen that the thickness of the mixed layer is decisive and the hydrogenation effect is large.
  • the coated copper wire for ball bonding of the present invention has high bonding reliability with respect to an aluminum electrode in a high temperature atmosphere while maintaining properties such as low electrical resistance and low cost due to a high purity copper core material. It can be widely applied for various uses.

Abstract

【課題】ボールボンディング用パラジウム被覆銅ワイヤにおいて、アルミニウム電極に対する接合信頼性を向上する。 【解決手段】パラジウム（Pd）中間層表面に厚さ5nm以下の極薄層からなる金（Au）層を形成し、水素を含む不活性雰囲気中で熱処理を行い、金極薄層に中間層のパラジウムが侵入して微細な金相とパラジウム相とが3次元成長するストランスキー・クラスタノフ成長により、金－パラジウム混在層を形成する。　熱処理過程でパラジウムは水素を吸収し、熱処理後急冷することにより、上記混在層のパラジウムを安定化し、溶融ボール形成時に早期に溶融してワイヤ端面を被覆する金に伴って端面に達したパラジウムが溶融して溶融ボール表面層に均一微細に分散して、アルミニウムとの接合界面におけるアルミニウムの酸化を抑制する。

Description

Pd被覆銅ボールボンディングワイヤ
　本発明は、半導体素子上の電極と回路配線基板の配線とをボールボンディングで接続するために用いられるPd被覆銅ワイヤに関する。
　現在、半導体素子上の電極と外部端子との間をボールボンディングで接合するボンディングワイヤとして、線径１５～３０μm程度の金線が主に使用されている。しかしながら、近年の金地金価格の高騰によってこれまでの高純度４Ｎ系(純度が９９．９９質量％以上)の金線に替わり、線径１０～２５μm程度の銅線に利用が注目されている。
　この銅線も金線と同様の利用分野が考えられており、例えば、実装関係では、現行のリードフレームを使用したＱＦＰ（Quad Flat Packaging）に加え、基板、ポリイミドテープ等を使用するＢＧＡ（Ｂall Grid Array）、ＣＳＰ（Chip Scale Packaging)等の新しい形態への応用が検討され、ボールボンディングで接合する際のループ性、接合性、量産使用性等をより向上したボンディングワイヤが要請されている。
他方、銅線のボンディングワイヤの接合相手となる材質も金線の場合と同様であって、シリコン基板上の配線、電極材料では、従来のアルミニウム（Ａｌ）合金パッドに加えて、より微細配線に好適な高純度の銅（Ｃｕ）が実用化されている。また、リードフレーム上には銀（Ａｇ）メッキ、金（Ａｕ）メッキ、更にニッケル（Ｎｉ）メッキ上のパラジウム（Ｐｄ）メッキ等が施されており、また、樹脂基板、テープ等の上には、銅（Ｃｕ）配線が施され、その上に金（Ａｕ）等の貴金属元素及びその合金の膜が施されている場合が多い。こうした種々の接合相手に応じて、銅（Ｃｕ）ワイヤの接合性、接合部信頼性を向上することが求められる。
当初は高純度３Ｎ～６N系（純度が９９．９質量％以上～純度９９．９９９９質量％以上）の銅（Ｃｕ）線の利用が考えられた。しかし、銅線は酸化しやすい欠点があった。このため、ＣｕやＣｕ－Ｓｎ等の芯材の外周に０．００２～０．５μｍのＰｄ、Ｐｄ－Ｎｉ、Ｐｄ－Ｃｏ等の被覆層を設け、耐食性並びに強度を改良することができる構造が提案されている（特許文献１参照）。更に、このパラジウム（Ｐｄ）層に金（Ａｕ）等の貴金属層を被覆したボンディングワイヤが提案されている（特許文献２～特許文献４参照）。
しかし、これらの銅線を８０℃～２００℃の高温下の環境に置かれる半導体用途に用いようとした場合、アルミニウム（Ａｌ）等のパッドにうまくボールボンディングできたとしても、パッドとワイヤの接合界面からアルミニウム（Ａｌ）の酸化物が成長して接合界面が剥離することがあった。
他方、これに対してパッドとワイヤの接合界面にパラジウム（Ｐｄ）を分散させることができれば、アルミニウム（Ａｌ）酸化物の成長を抑制することができることが知られている（非特許文献１）。
実開昭６０－１６０５５４号公報 特開昭６２－９７３６０号公報 特開２００５－１６７０２０号公報 ＷＯ２０１１－１３５２７号公報 特許第３０２４５８４号公報
２００６年７月SEIテクニカルレビュー第169号　P47-51、「ハイブリッドボンディングワイヤーの開発」改森信悟　ほか２名、
パラジウム被覆銅ワイヤそのものは、従来より知られているが、主として酸化されやすい銅芯材の酸化防止やパラジウム被覆リードフレームに対するウエッジボンディングを行うものに用いられるものであって、単に、このようなパラジウム被覆層を設けても溶融ボール形成時に芯材の銅中に取り込まれて肝心のボール表面層に必要な濃度のパラジウムが分散したものとはならなかった。
一般にこれらのパラジウム被覆層は、素材のパラジウムが高価であるため、比較的薄く、銅（Ｃｕ）の芯材に対して高価なパラジウム（Ｐｄ）被覆層の占める割合は、２０分の１以下と少ないので、銅（Ｃｕ）ボールの形成と同時にパラジウム（Ｐｄ）が銅（Ｃｕ）中に拡散してしまい、溶融ボールの外側にパラジウム（Ｐｄ）を均一微細に分散させるに至らない。
また、特許文献４記載のパラジウム被覆銅ワイヤはパラジウム被覆リードフレームとのウエッジ接合を行うためのワイヤであるが、パラジウム被覆層の表面に３～８０nmの厚さを有する金とパラジウムとを含む合金層を形成することが行われている。
　しかしながら、このようにパラジウムの合金化を行ってもやはり、芯材の銅に拡散してしまい、溶融ボール表面のパラジウム分散層を形成することはできなかった。
　本発明者らは、芯材の銅（Ｃｕ）よりも融点の低い金（Ａｕ）を芯材の銅（Ｃｕ）の外側に配置し、金（Ａｕ）を誘い水として芯材の銅（Ｃｕ）を先に溶融し、銅（Ｃｕ）よりも融点の高いパラジウム（Ｐｄ）を後から溶融することによって溶融ボールの外側にパラジウム（Ｐｄ）を均一微細に分散させることができる構造にしようとするものである。このため、パラジウム（Ｐｄ）上に金（Ａｕ）の被覆層を形成した後、中程度の熱処理を施して金（Ａｕ）とパラジウム（Ｐｄ）を合金化することなく、いわゆるストランスキー・クラスタノフ成長を利用して、金（Ａｕ）の被覆層中にパラジウム（Ｐｄ）を３次元成長させ、金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層を形成させるものである。
このストランスキー・クロスタノフ型の成長により形成される組織は、極めて薄い金属層で発生し、下地金属から成長したアイランド構造を形成して表面層の平坦性を損なうことで知られている（特許文献５）が、本発明の構成においてこのストランスキー・クラスタノフ成長により形成される組織は、パラジウム被覆層上に形成される金層が極めて薄く、厚さ数nm以下の金層中でパラジウムが３次元成長して形成され、微細なパラジウム相と金相とが混在する構造となる。
金（Ａｕ）とパラジウム（Ｐｄ）は、本来すべての割合で完全に混ざり合い、結晶格子が約４．９％だけわずかに食い違うだけなので、金（Ａｕ）／パラジウム（Ｐｄ）二層の密着性は良く、金（Ａｕ）層が極薄の場合にはストランスキー・クラスタノフ成長を利用して金（Ａｕ）層表面にパラジウム（Ｐｄ）下地層を析出させることができる。
ところで、この金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層は不安定であり、金（Ａｕ）とパラジウム（Ｐｄ）の純度が高くなればなるほど合金化しやすくなる。そこで、本発明者らは、この金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層を安定化させるため、高温で水素ガスをパラジウム（Ｐｄ）表面に接触させてパラジウム（Ｐｄ）中に水素原子を吸収させ、この混在層をより安定化させることにした。熱処理後、急冷する水溶液にエタノール等のアルコールを混入しておくと、アルコールが熱分解した水素原子を急冷されたパラジウム（Ｐｄ）表層が取り込むので、更に好ましい。
本発明者らは、連続伸線後の最終熱処理を適当に調整することによって、上記のストランスキー・クラスタノフ成長を利用する熱処理と銅（Ｃｕ）ボンディングワイヤの伸び・引張り強度を調質する最終アニールとを併せて行うことにし、短時間の最終熱処理に続いてそのままワイヤを液冷することによって、より安定化した金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層の構造を確実に保持することにした。
さらに、本発明者らは、連続伸線後の最終熱処理を適当に調整することによって、上記のストランスキー・クラスタノフ成長を利用する熱処理とパラジウム（Ｐｄ）中に水素原子を吸収させる高温処理と銅（Ｃｕ）ボンディングワイヤの伸び・引張り強度を調質する最終アニールとをまとめて行うことにし、短時間最終熱処理に続いてそのままワイヤを液冷することによって、より安定化した金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層の構造を確実に保持することにした。
本発明は、上記の課題を解決することを目的とし、具体的には以下の構成を特徴とする。
（１）　本発明のボールボンディング用被覆銅ワイヤは、銅（Ｃｕ）または銅合金からなる芯材、パラジウム（Ｐｄ）からなる中間被覆層からなる表面被覆された線径が１０～２５μｍのボールボンディング用被覆銅ワイヤにおいて、前記中間被覆層は純度９９質量％以上のパラジウム（Ｐｄ）であり、さらに該中間層上に金（Ａｕ）の最上層を形成してその該中間被覆層の芯材とは反対側の接合界面上に前記パラジウム（Ｐｄ）と純度９９．９質量％以上の金（Ａｕ）が熱成長によって入り乱れた、走査電子顕微鏡観察による断面の平均厚さが５ｎｍ以下の混在層を有し、かつ、この混在層のパラジウム表面が水素拡散処理されていることを特徴とする。
本発明のボールボンディング用被覆銅ワイヤの具体的な態様は、次のとおりである。
（２）上記混在層断面の平均厚さが３ｎｍ以下である上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（３）上記混在層断面の平均厚さが１ｎｍ以下である上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（４）上記パラジウム（Pd）が湿式メッキされた上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（５）上記混在層が強伸線加工された金（Ａｕ）およびパラジウム（Ｐｄ）が被覆された銅ワイヤに対して、ストランスキー・クラスタノフ成長により、金（Ａｕ）の被覆層中にパラジウム（Ｐｄ）を3次元成長させ、金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層を形成させた上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（６）上記混在層が強伸線加工された金（Ａｕ）およびパラジウム（Ｐｄ）が被覆された銅ワイヤに対して４５０℃～７００℃の水素含有不活性雰囲気下で行われた上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（７）上記金（Ａｕ）が室温でマグネトロンスパッタされた上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（８）中間被覆層が湿式メッキされたパラジウム（Ｐｄ）である上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（９）上記芯材の銅（Ｃｕ）が純度９９．９９９質量％以上の銅（Ｃｕ）である上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（１０）上記芯材の銅（Ｃｕ）が純度９９．９９９９質量％以上の銅（Ｃｕ）である上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（１１）上記芯材の銅合金が0.1～５００質量ｐｐｍリン（P）および残部銅（Ｃｕ）からなる上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（１２）上記芯材の銅合金が０．５～９９質量ｐｐｍのジルコニウム（Ｚｒ）、スズ（Ｓｎ）、バナジウム（Ｖ）、ホウ素（Ｂ）およびチタン（Ｔｉ）のうちの少なくとも1種を総量で0.５～９９質量ppm含み、および残部が純度９９．９質量％以上の銅（Ｃｕ）からなる上記（１）に記載のボールボンディング用被覆銅ワイヤ。
（１３）上記芯材の銅合金が０．５～９９質量ｐｐｍのジルコニウム（Ｚｒ）、スズ（Ｓｎ）、バナジウム（Ｖ）、ホウ素（Ｂ）およびチタン（Ｔｉ）のうちの少なくとも1種を総量で0.５～９９質量ppm、と０．１～５００質量ｐｐｍリン（P）とを含み、および残部が純度９９．９質量％以上の銅（Ｃｕ）からなる上記（１）に記載のボールボンディング用被覆銅ワイヤ。
本発明によれば、第一ボンドの溶融ボール形成時に、まず溶けやすい金（Ａｕ）を誘い水にして芯材の銅（Ｃｕ）を溶融させ、次いで銅（Ｃｕ）に接しているパラジウム（Ｐｄ）が溶融し、最後に水素原子を吸蔵した表面のパラジウム（Ｐｄ）が溶融するようにしたので、パッド上のアルミニウム（Ａｌ）界面と相接する溶融ボールの表面にパラジウム（Ｐｄ）を均一微細に分散させることができた。
また、本発明は、純度９９．９質量％以上の金（Ａｕ）の膜厚と最終熱処理を適当に調整することによって、金（Ａｕ）とパラジウム（Ｐｄ）の合金層ではなく、金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層を形成することができた。
さらに、最終熱処理中に水素ガスを導入することにより、金（Ａｕ）表面層に顔を出したパラジウム（Ｐｄ）と水素分子を反応させ、水素原子をパラジウム（Ｐｄ）内に吸収させて、金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層をより確実に安定化させることができた。
本発明のワイヤ構造では、第一ボンドの溶融ボール形成時に、先ずストランスキー・クラスタノフ構造の最表層中で最も融点の低い（１０６４℃）金（A）が溶融を始め、同じ層中に混在するパラジウムを伴って流動し、中間層のパラジウム層からワイヤ端面に至って露出した銅芯材の表面を覆う。
銅（Cu）は、金（Au）、パラジウム（Ｐｄ）よりも融点が高い（１０８５℃）が、ワイヤ端面で溶融した金（Au）に直接接して速やかに溶融し、いわば溶融した金（Au）を誘い水として速やかに溶融して、ボールを形成する。
ワイヤの構造中で最も融点の高い（１５５５℃）パラジウム（Pd）は、先に銅が溶融する間その周囲に一旦は薄い鞘状に残るが、銅の溶融ボールが形成する間に溶融しながら銅ボール中に取り込まれて拡散して行き、一方、先に溶融した金（Ａｕ）に伴ってワイヤ端面に至った混在層のパラジウムは吸蔵した水素原子により安定化されて合金化することなく最後に溶融し、ボール表面近傍に偏析して高いパラジウム（Pd）濃度を維持するものと考えられる。
本発明のボンディングワイヤは、上記の構造を有するので、高温に長期間放置してもワイヤとパッドとのボンディング接合界面にアルミニウム酸化物等が発生することがなく、長期間の高温安定性に優れている。
本発明の被覆銅ワイヤにおいて、超極薄の表面層の理論的な厚さの上限は１０nmであり、この上限は８nmが好ましく、より好ましくは上限７nm以下である。ここで、「理論的な厚さ」とは、このような超極薄の全表面層を直接測定することはきわめて困難であるため、連続伸線前の乾式メッキ時の表面層の厚さから比例計算で求めたものである。
「理論的な厚さ」を求めるときの比例定数は、ボンディングワイヤとしての連続伸線の終了後の線径を連続伸線の開始前のワイヤの直径で除した値である。実際の厚さは、高倍率の走査電子顕微鏡で直接観察することができるが、理論的な厚さで見てもナノオーダーであるため、およその平均膜厚しか得られない。薄ければ薄いほど、金（Ａｕ）とパラジウム（Ｐｄ）のストランスキー・クラスタノフ成長がおきやすい。逆に厚くなり過ぎると、金（Ａｕ）とパラジウム（Ｐｄ）の相互作用よりも金（Ａｕ）相互の相互作用が強くなってパラジウム（Ｐｄ）が金（Ａｕ）表面から顔を出すことができなくなり、水素原子をパラジウム（Ｐｄ）内に吸収させることができなくなる。
超極薄の金（Ａｕ）表面層は、付きまわり性および中間被覆層との接合性の観点から、スパッタリング法によるコーティングがよい。スパッタリング法によりコーティングされた金（Ａｕ）等は純度９９．９質量％以上であっても気体分子の衝突によって硬質となり、付きまわり性がよい。金（Ａｕ）等の表面層は、冷間での連続伸線時に相対的にさらに硬質となるパラジウム（Ｐｄ）析出物の微細な凹凸の接合界面内部まで入り込み、冷間伸線中にしっかり接合される。
なお、スパッタリング法によりコーティングされた金（Ａｕ）等の超極薄の表面層は、線材の円周方向の断面形状で幾何学的に均一な円形の膜とはならないが、溶融ボール形成時の表面張力によって芯材の銅（Ｃｕ）中に速やかに吸収され、表面層の不均一さは解消され、真球状の溶融ボールが形成される。
芯材の特に好ましい態様は、芯材が１～８０質量ｐｐｍリン（Ｐ）を含有し、残部が純度９９．９９９質量％以上の銅（Ｃｕ）から構成されている。リン（Ｐ）は微量でも、銅ワイヤの再結晶温度を上昇させ、ワイヤ自体の強度を硬くする効果があるからである。ここで、「純度９９．９９９質量％以上」とは、リン（Ｐ）および銅（Ｃｕ）以外の金属の不純物元素が０．００１質量％未満であることをいい、銅（Ｃｕ）中に存在する酸素や窒素や炭素などのガス状元素を除いたものをいう。芯材に所定量のリン（Ｐ）が含まれると、第一ボンディングにおいて溶融した銅ボールが凝固していく過程で、銅（Ｃｕ）ボールの脱酸素作用をする。銅（Ｃｕ）の純度９９．９９９質量％以上あれば、リン（Ｐ）は１質量ｐｐｍ以上の範囲で脱酸素作用をし、８０質量ｐｐｍ以下の範囲であれば、伸線時に銅（Ｃｕ）芯材が加工硬化することはない。このリン〈Ｐ〉の脱酸素効果により、芯材の銅（Ｃｕ）に酸化していた部分があっても、溶融ボールの表面層近傍でリン（Ｐ）が濃縮することにより銅（Ｃｕ）の酸化を分断消去させることで、溶融ボール形成時に芯材の銅（Ｃｕ）が酸化していた部分の影響をないようにすることができる。
中間層は、パラジウム（Ｐｄ）から構成される。パラジウム（Ｐｄ）の融点（１５５４℃）は、銅（Ｃｕ）の融点(約１０８５℃)又は微量元素を添加した銅合金の融点のいずれもよりも高い。このため芯材の銅（Ｃｕ）または銅合金が球状の溶融ボールを形成していく最初の段階では、パラジウム（Ｐｄ）が薄皮となって溶融ボールの側面からの酸化を防止する。
パラジウム中間層の純度は、溶融ボールの真球性にさほど影響を与えず、薄くても厚くても偏芯とならないが、連続伸線する上から純度９９質量％以上のパラジウム（Ｐｄ）が必要である。中間層の厚さは適宜定まるが、パラジウム（Ｐｄ）中間層が厚くなればなるほど銅（Ｃｕ）または銅合金の芯材の劣化が遅くなる傾向がある。
パラジウム（Ｐｄ）中間被覆層の形成方法には、乾式メッキや湿式メッキを利用することができる。乾式メッキにはスパッタ法、イオンプレーティング法、真空蒸着等を利用することができる。不純物の混入を避ける観点からは乾式メッキが好ましいが、断面が均一な円環形状を得るには湿式メッキがよい。
銅（Ｃｕ）は純度が高いので、その純度を維持するためパラジウム（Ｐｄ）の電解メッキ浴もハロゲンイオンや硫酸イオンの含まないアンモニア性水溶液やシアン系水溶液のものが好ましい。また、高分子化合物や金属塩の光沢剤は溶融ボールの真球性に悪影響を与えるので、膜成分としては含まないのが好ましい。銅（Ｃｕ）等の芯材に電解メッキされた膜はその後の連続伸線によって強圧縮加工されるので、膜性状は膜成分ほど重要ではない。膜成分にイオウ（Ｓ）が存在すると、溶融ボールの形成時に銅（Ｃｕ）に混入して溶融ボールを加工硬化させるおそれがあるからである。
パラジウム（Ｐｄ）電解メッキ浴としては、パラジウムｐ－ソルト（Ｐｄ（ＮＨ３）２（ＮＯ２）２）、亜硝酸アンモニウムおよび硝酸カリウム、またはパラジウムｐ－ソルト（Ｐｄ（ＮＨ３）２（ＮＯ２）２）、硝酸アンモニウムおよびアンモニア水の弱アルカリ性アンモニア性水溶液、Ｐｄ（ＮＨ３）２（ＣＯＯ）２　および（ＮＨ４）２ＨＰＯ４　の中性アンモニア性水溶液（米国特許第Ｓ４７１５９３５号）などが利用できる。パラジウムｐ－ソルトを用いた浴では、PHが高いほど、析出物の粒径は大きくなる傾向にあった。パラジウム（Ｐｄ）を乾式メッキする場合、スパッタ膜等の異常析出を防ぐため銅（Ｃｕ）の純度は９９．９９質量％よりも９９．９９９質量％以上のものが好ましい。
なお、溶融ボールの形成時に金（Ａｕ）表面極薄層が銅（Ｃｕ）芯材に溶け込むタイミングを調整するため、パラジウム（Ｐｄ）メッキをする前にパラジウム（Ｐｄ）や白金（Ｐｔ）やニッケル（Ｎｉ）等のストライクメッキ（極薄メッキ）を施すこともできる。
アーク放電による溶融ボールの形成において各被覆層中の金属の融点は非常に重要である。ボンディングワイヤの大部分は高純度の銅（Ｃｕ）の芯材が占めるので、銅（Ｃｕ）の融点（約１０８５℃）が基準になる。芯材の高純度の銅（Ｃｕ）は、還元性雰囲気中でアーク放電によって完全な真球形状となることが知られている。
また、パラジウム（Ｐｄ）を被覆した高純度銅（Ｃｕ）の芯材も非酸化性雰囲気中でアーク放電によって真球形状となることが知られている。パラジウム（Ｐｄ）の融点（約１５５５℃）は銅（Ｃｕ）の融点（約１０８５℃）よりも高いので、非酸化性雰囲気中で銅（Ｃｕ）が真球形状となるのに引きずられて真球形状となるものと考えられる。
しかし、高純度の金（Ａｕ）のボンディングワイヤは雰囲気を問わずアーク放電により溶融ボールを形成すると真球状の溶融ボールが得られるにもかかわらず、高純度の金（Ａｕ）を高純度銅（Ｃｕ）の芯材に直接被覆したボンディングワイヤは、槍状になってしまい、真球形状のボールが得られない。金（Ａｕ）の融点（約１０６４℃）は、銅（Ｃｕ）の融点（約１０８５度）よりも低いので、銅（Ｃｕ）が球状の溶融ボールを形成していく段階で、低融点の金（Ａｕ）表面層が銅（Ｃｕ）よりも早く早期に融解してワイヤ端面をすばやく包もうとするが、高融点のパラジウム（Ｐｄ）が邪魔となる。その結果、低融点の金（Ａｕ）は銅（Ｃｕ）中への拡散が優先して、溶融銅（Ｃｕ）に吸収されていき、その後パラジウム（Ｐｄ）が溶融していくものと考えられる。銅（Ｃｕ）中の微量添加元素は、溶融現象にほとんど影響しない。
このように一般的なパラジウム被覆銅ワイヤにおいて、最表層の金（Ａｕ）層の厚さが溶融ボールの真球性に影響を及ぼすが、本発明においては前記したように先に溶融した金（Ａｕ）はワイヤ端面において銅（Ｃｕ）の融解を促進するが、本発明における金（Ａｕ－パラジウム（Ｐｄ）混在層は数ｎｍのオーダーであって、金（Ａｕ）の量が微量であるためその影響は抑制され、溶融ボールの真球性に悪影響を及ぼすことはない。
パラジウム（Ｐｄ）中間層の湿式メッキは、レべリング剤や光沢剤などが含まれていないので、不規則な粒状に析出する傾向にある。また、その乾式メッキは高純度の銅（Ｃｕ）の芯材の結晶面に沿って層状に析出する傾向にある。いずれの場合もワイヤ断面は完全な円輪形状でないが、表面層の厚みが１nm以上あれば十分である。
高純度の金（Ａｕ）等の表面層は、スパッタリングによる付きまわりだけでなく、スパッタリング中にワイヤを軸中心に回転させながら移動したり、スパッタリング中にワイヤを往復させて移動したり、ワイヤの両側からスパッタしたりして、高純度の金（Ａｕ）を中間被覆層上により均一な厚さで析出させることができる。高純度の金（Ａｕ）等の表面層は、展延性が良いので、ダイヤモンドダイスのダイス穴形状にしたがって最終線径まで連続伸線加工することができる。連続伸線加工中に金（Ａｕ）等の表面層と中間被覆層との界面の隙間は埋められ、パラジウム（Ｐｄ）中間層の湿式メッキに異常析出等があっても機械的に表面層を突き破って析出するようなことはなく、ボンディングワイヤの表面は金-（Ａｕ）等が全面的に被覆されている。
連続伸線は冷却液中で行う湿式伸線が良い。最表層の被覆層が薄いため乾式伸線では強圧縮作用に伴う熱によって超極薄の表面層の金（Ａｕ）が銅（Ｃｕ）の芯材中へ拡散して消失してしまうおそれがあるからである。ダイヤモンドダイスと金（Ａｕ）との摩擦抵抗を下げるため、市販の界面活性剤を添加した金属潤滑液を水やアルコール等の希釈液で希釈して使用するほか、エチルアルコール、メチルアルコールまたはイソプロピルアルコールだけを含有した水溶液などの溶液中で連続伸線するのが良い。
　以下、実施例について説明する。
表１に記載の銅（Ｃｕ）インゴットから５００μｍの線径まで伸線加工した銅ワイヤを芯材とし、そのワイヤ表面に通常の方法でパラジウム（Ｐｄ）中間層の電解メッキを２．０μｍ析出させた。このパラジウム（Ｐｄ）メッキ浴は、中性のジニトロジアンミンパラジウム浴に１０ｇＷ／ｌのリン酸塩を添加したものを使用し、得られたパラジウム（Ｐｄ）の純度は９９％であった。次いで、室温で純度９９．９９質量％の金（Ａｕ）をマグネトロンスパッタし、０．０８μｍ析出させた。なお、メッキ厚はオージェ電子分光法（ＡＥＳ）で測定した。
その後、この被覆銅ワイヤを最終径の１７μｍまでダイス伸線した。金（Ａｕ）の理論的膜厚は０．００２７μｍである。次いで、加工歪みを取り除き、伸び値が１０％程度になるように所定の最終熱処理を施した。最終熱処理条件は、５％水素＋窒素雰囲気で７００℃の熱処理炉の長さ５０ｃｍを８ｍ／秒で通過させ、１０％エタノール水溶液（２０℃）中で冷却した。
Figure JPOXMLDOC01-appb-T000001
実施例１と同じ条件で製造した被覆銅ワイヤを５％水素＋窒素雰囲気で６００℃の熱処理炉を５ｍ／秒で通過させ、純水（４０℃）中で冷却した。
　〔比較例１〕
純度９９質量％の金（Ａｕ）を純金メッキ（日本エレクトロプレーティングエンジニアーズ株式会社製のオートロネクスシリーズGVC-S）浴により０．００２７μｍ（理論的膜厚０．００２７μｍ）析出させ、窒素雰囲気で５５０℃の熱処理炉を５ｍ／秒で通過させた以外は、実施例１と同様にした。
　〔比較例２〕
最終熱処理条件を、５％水素＋窒素雰囲気で８００℃の熱処理炉を８ｍ／秒で通過させた以外は、実施例２と同様にした。
　以上の実施例及び比較例のワイヤについて、製造条件を表２に示す。
Figure JPOXMLDOC01-appb-T000002
以上の実施例及び比較例の線材について、アルミ電極上にボンディング及びステッチ接合を行い、高温信頼性評価、接合強度性評価、軸上偏芯評価を行った。
それらの接合条件、試験条件及び評価結果を次に示す。
ボンディングワイヤの接続には、市販の自動ワイヤボンダ((株)K&S社製の超音波熱圧着ワイヤボンダ「MAXμｍ Ultra(商品名)」)を使用しボール/ステッチ接合を行った。溶融ボールはMaxμｍ plus Copper Kitを用いて、流量0.5(l/min)で4体積％水素と残部窒素からなる混合ガスを使用して、ガス雰囲気中でアーク放電によりワイヤ先端にボールを作製した。
それをシリコン基板上の0.8μmアルミニウム(Al-0.5％Cu)電極膜に接合し、ワイヤ他端を4μmの銀(Ag)メッキした200℃のリードフレーム(材質は42アロイ、膜厚は150μｍ)上にステッチ接合した。キャピラリーはSPT社製を使用し、溶融ボールに関するワイヤボンダの設定値は、EFO Fire ModeをBal Sizeとし、FAB Sizeは実際の溶融ボール径がワイヤ径の2倍となるように調整した。圧着径はワイヤ径の2.5倍となるように、ボンディング時の接合条件を調整した。
高温信頼性評価
上記の条件でボンディングされ、1st接合部がアルミニウム(Al-0.5％Cu)電極膜に、2nd接合部が銀(Ag)メッキしたリードフレームにボンディングしたサンプルを用いた。当該サンプルの1st接合部のパッド形状は角90μmから成り、100μmピッチで配置されている。また、隣接する1st接合部は一部電気的に通電するように回路設計されている。ボンディング後はハロゲンが含まれる市販の封止樹脂で樹脂モールドした後、余分なタイバー等を切断し、その後温度175℃で2時間キュアし、最終的に温度220℃の高温加熱炉で任意の時間放置した。電気抵抗は、ＫＥＩＴＨＬＥＹ社製の製品名「ソースメーター（型式２００４）」を用い、専用のＩＣソケットおよび専用に構築した自動測定システムでおこなった。測定方法はいわゆる直流四端子法で測定している。測定用プローブから隣接する外部リード間（ＩＣチップ上のパッドが短絡した対を選択）に一定電流を流し、プローブ間の電圧が測定される。電気抵抗は外部リード１００対（２００ピン）について、放置前と放置後に電気抵抗測定を行い電気抵抗の上昇率が20％以上となるものを不良とした。良否判定は各サンプルの不良率が50％に達するまでの時間が長いものを良好とした。時間が200時間以上であれば実用上の大きな問題はないと判断して◎印、150時間以上200時間未満であれば○印、100時間以上～150時間未満である場合に△印、100時間未満である場合に×印で表記した。
これらの結果を表３に示す。
Figure JPOXMLDOC01-appb-T000003
接合強度性評価
接合強度性評価は、上記のアルミニウム(Al-0.5％Cu)電極膜は使用せずに、ワイヤ両端を4μmの銀(Ag)メッキした200℃のリードフレーム(材質は42アロイ、膜厚は150μｍ)上にボール/ステッチ接合した。3920本のワイヤをボンディングし、不圧着回数が０～１本を◎、２～３本を○、４～２０本を△、２１本以上を×とした。
これらの結果を表４に示す。
Figure JPOXMLDOC01-appb-T000004
軸上偏芯評価
軸上偏芯評価は、(株)K&S社製の超音波熱圧着ワイヤボンダ「MAXμｍ Ultra(商品名)」を使用しLoop
ParameterをFAB Modeとして連続的にFABを作成して評価を実施した。ボンディングは、厚み4μmの銀(Ag)メッキした200℃のリードフレーム上へ連続ボンディングし、アルミニウム(Al-0.5％Cu)電極膜は使用しなかった。なお、その他のワイヤボンディングに関する設定値は、上記のアルミニウム(Al-0.5％Cu)電極膜のダメージ評価と同様に行なった。判定は、接合前の溶融ボール形状を200個観察して、軸上偏芯と寸法精度が良好であるか等を判定した。ワイヤに対するボール位置の芯ずれが5nm以上ある個数を測定し、芯ずれが1個以下である場合は、ボール形成は良好であるため◎印、2～4個であれば実用上の大きな問題はないと判断して○印、5～9個である場合に△印、10個以上である場合に×印で表記した。
これらの結果を表５に示す。
Figure JPOXMLDOC01-appb-T000005
以上の各表の評価結果から、実施例１と実施例２とは表２に示すように熱処理条件及び冷却条件に相違があり、混合層の厚さではほとんど差がないが、評価において若干低下する。これらに対して比較例１は混在層の厚さに差がないが、熱処理雰囲気中に水素添加を行っていないため好結果を得られていない。また、Au被覆方法がAuメッキであるため、湿式メッキと比べて膜の密着性・緻密性が低く、局所的に不均一な膜厚が存在することや、添加剤やPH調整材などの不純物が多く含有されるため好結果が得られない。
また、比較例２は熱処理温度が高すぎるため混在層の厚さが本発明範囲を大きく超えて肥大し、効果が得られていない。
即ち、混在層の厚さが決定的であり、また水素添加効果が大きいことが解る。
　本発明のボールボンディング用被覆銅ワイヤは、高純度銅芯材による低電気抵抗性、安価であることなどの性質を維持しつつ、高温雰囲気中でアルミニウム電極に対して高い接合信頼性を有しており、各種の用途に向けて広く適用することができる。
 

Claims (14)

  1. 銅（Ｃｕ）または銅合金からなる芯材、純度９９質量％以上のパラジウム（Ｐｄ）からなる中間被覆層からなる表面被覆された線径が１０～２５μｍのボールボンディング用被覆銅ワイヤにおいて、最上層として、純度９９．９質量％以上の金（Ａｕ）層が水素含有雰囲気中で熱処理されることにより、上記中間層から該金（Au）層中にパラジウム（Pd）が熱成長して該金（Au）層表面に露出すると共に該パラジウム（Pd）が水素拡散処理された、走査電子顕微鏡観察による断面の平均厚さが５ｎｍ以下の金（Au）－パラジウム（Pd）混在層を形成していることを特徴とするボールボンディング用被覆銅ワイヤ。
  2. 上記混在層断面の平均厚さが３ｎｍ以下であることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  3. 上記混在層断面の平均厚さが１ｎｍ以下であることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  4. 上記パラジウム（Pd）が湿式メッキされた請求項１に記載のボールボンディング用被覆銅ワイヤ。
  5. 上記混在層が強伸線加工された金（Ａｕ）およびパラジウム（Ｐｄ）が被覆された銅ワイヤに対して、ストランスキー・クラスタノフ成長を利用して、金（Ａｕ）の被覆層中にパラジウム（Ｐｄ）を3次元成長させ、金（Ａｕ）とパラジウム（Ｐｄ）が入り乱れた混在層を形成させるものである請求項１に記載のボールボンディング用被覆銅ワイヤ。
  6. 上記混在層が強伸線加工された金（Ａｕ）およびパラジウム（Ｐｄ）が被覆された銅ワイヤに対して４５０℃～７００℃の水素含有不活性雰囲気下で行われたものである請求項１に記載のボールボンディング用被覆銅ワイヤ。
  7. 上記金（Ａｕ）が室温でマグネトロンスパッタされたものである請求項１に記載のボールボンディング用被覆銅ワイヤ。
  8. 中間被覆層が湿式メッキされたパラジウム（Ｐｄ）である請求項１に記載のボールボンディング用被覆銅ワイヤ。
  9. 上記芯材の銅（Ｃｕ）が純度９９．９９９質量％以上の銅（Ｃｕ）であることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  10. 上記芯材の銅（Ｃｕ）が純度９９．９９９９質量％以上の銅（Ｃｕ）であることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  11. 上記芯材の銅合金が0.1～500質量ppmリン（P）および残部銅（Ｃｕ）からなることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  12. 上記芯材の銅合金が0.５～９９質量ppmのジルコニウム（Ｚｒ）、スズ（Ｓｎ）、バナジウム（Ｖ）、ホウ素（Ｂ）およびチタン（Ｔｉ）のうちの少なくとも1種を総量で0.５～９９質量ppm含み、および残部が純度９９．９質量％以上の銅（Ｃｕ）からなることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  13. 上記芯材の銅合金が0.５～９９質量ppmのジルコニウム（Ｚｒ）、スズ（Ｓｎ）、バナジウム（Ｖ）、ホウ素（Ｂ）およびチタン（Ｔｉ）のうちの少なくとも1種を総量で0.５～９９質量ppm、と0.1～500質量ppmリン（P）とを含み、および残部が純度９９．９質量％以上の銅（Ｃｕ）からなることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
  14. 上記芯材の銅合金が0.５～９９質量ppmのジルコニウム（Ｚｒ）、スズ（Ｓｎ）、バナジウム（Ｖ）、ホウ素（Ｂ）およびチタン（Ｔｉ）のうちの少なくとも1種を総量で0.５～９９質量ppm、と１～８０質量ppmリン（P）とを含み、および残部が純度９９．９質量％以上の銅（Ｃｕ）からなることを特徴とする請求項１に記載のボールボンディング用被覆銅ワイヤ。
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WO2016189758A1 (ja) * 2015-05-26 2016-12-01 日鉄住金マイクロメタル株式会社 半導体装置用ボンディングワイヤ
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