WO2016006326A1 - 半導体装置用ボンディングワイヤ - Google Patents

半導体装置用ボンディングワイヤ Download PDF

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Publication number
WO2016006326A1
WO2016006326A1 PCT/JP2015/064417 JP2015064417W WO2016006326A1 WO 2016006326 A1 WO2016006326 A1 WO 2016006326A1 JP 2015064417 W JP2015064417 W JP 2015064417W WO 2016006326 A1 WO2016006326 A1 WO 2016006326A1
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WIPO (PCT)
Prior art keywords
bonding
bonding wire
wire
ball
total
Prior art date
Application number
PCT/JP2015/064417
Other languages
English (en)
French (fr)
Inventor
哲哉 小山田
宇野　智裕
博之 出合
大造 小田
Original Assignee
新日鉄住金マテリアルズ株式会社
日鉄住金マイクロメタル株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 新日鉄住金マテリアルズ株式会社, 日鉄住金マイクロメタル株式会社 filed Critical 新日鉄住金マテリアルズ株式会社
Priority to KR1020167005271A priority Critical patent/KR101678806B1/ko
Priority to CN201580001736.3A priority patent/CN105492637B/zh
Priority to EP15819755.8A priority patent/EP3029167B1/en
Priority to US14/915,189 priority patent/US10381320B2/en
Priority to SG11201601519YA priority patent/SG11201601519YA/en
Priority to JP2015532629A priority patent/JP5839763B1/ja
Publication of WO2016006326A1 publication Critical patent/WO2016006326A1/ja
Priority to PH12016500383A priority patent/PH12016500383A1/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C5/00—Alloys based on noble metals
    • C22C5/06—Alloys based on silver
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C5/00—Alloys based on noble metals
    • C22C5/06—Alloys based on silver
    • C22C5/10—Alloys based on silver with cadmium as the next major constituent
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Definitions

  • the present invention relates to a bonding wire for a semiconductor device used for connecting an electrode on a semiconductor element and a wiring of a circuit wiring board such as an external lead.
  • bonding wires fine wires having a wire diameter of about 15 to 50 ⁇ m are mainly used as bonding wires for semiconductor devices (hereinafter referred to as bonding wires) for bonding between electrodes on semiconductor elements and external leads.
  • the bonding wire bonding method is generally a thermocompression bonding method using ultrasonic waves, and a general-purpose bonding apparatus, a capillary jig for connecting the bonding wire through the inside, or the like is used.
  • the tip of the wire is heated and melted by arc heat input to form a ball by surface tension, and then this ball portion is bonded to the electrode of the semiconductor element heated within a range of 150 to 300 ° C.
  • the electrode on the semiconductor element that is the bonding partner of the bonding wire is an electrode structure in which an alloy film mainly composed of Al is formed on a Si substrate, and the electrode on the external lead side is subjected to Ag plating, Pd plating, or the like A structure is often used.
  • the bonding wire is required to have excellent ball formability, ball bondability, wedge bondability, loop formability, and the like.
  • Au has been mainly used as a bonding wire material that comprehensively satisfies these required performances.
  • the development of bonding wires using a relatively inexpensive material as an alternative to Au has been actively performed against the background of the rising price of Au.
  • the feature of this bonding wire is that the performance of the bonding wire is comprehensively improved mainly by suppressing the oxidation of Cu, and it is used in the state-of-the-art LSI (Large Scale Integration) field.
  • bonding wires using Ag have a problem that bonding reliability and loop stability are low in high-density mounting.
  • the joint reliability evaluation is performed for the purpose of evaluating the joint life in the actual use environment of the semiconductor device. In general, a high temperature standing test and a high temperature and high humidity test are used for evaluating the bonding reliability.
  • the Ag bonding wire has a problem that the life of the ball bonded portion in a high temperature and high humidity test is inferior to a bonding wire using Au (hereinafter referred to as an Au bonding wire).
  • Au bonding wire In high-density mounting, since small ball bonding is performed, the area that contributes to bonding is reduced, making it even more difficult to ensure the life.
  • a defect called a spring failure becomes a problem.
  • the spring failure causes a short circuit due to the bonding wires coming into contact with each other due to a phenomenon in which the loop is bent in the bonding wire bonding process.
  • the interval between adjacent bonding wires is narrow, suppression of spring failure is strongly required.
  • Spring failure is more likely to occur as the strength of the wire is lower, and this is often a problem in high-density mounting where the wire diameter is reduced.
  • Patent Document 1 discloses that one or more of Pt, Pd, Cu, Ru, Os, Rh, and Ir is 0.1 in total. Up to 10 wt%, Pt 10 wt% or less, Pd 10 wt% or less, Cu 5 wt% or less, Ru 1 wt% or less, Os 1 wt% or less, Rh 1 wt% or less, Ir There is disclosed a bonding wire that is 1% by weight or less and the balance is made of Ag and inevitable impurities.
  • Patent Document 2 discloses a ternary alloy bonding wire composed of Ag having a purity of 99.99% by mass or more, Au having a purity of 99.999% by mass or more, and Pd having a purity of 99.99% by mass or more. 4 to 10% by mass, Pd 2 to 5% by mass, oxidizing non-noble metal added element 15 to 70 ppm by mass, and the balance consisting of Ag.
  • the bonding wire is subjected to annealing heat treatment before continuous die drawing and continuously
  • an Ag—Au—Pd ternary alloy bonding wire for semiconductor elements which is subjected to tempering heat treatment after die drawing and ball bonding in a nitrogen atmosphere.
  • Patent Document 3 is characterized in that the tensile strength measured in the temperature atmosphere after heating for 15 to 25 seconds in a temperature atmosphere of 523 K is higher than the 0.2% proof stress measured in the temperature atmosphere of 298 K.
  • a bonding wire is disclosed.
  • the Ag bonding wire disclosed in the above patent document cannot satisfy the required performance for bonding reliability, spring failure, and chip damage required for high-density mounting.
  • HAST Highly Accelerated temperature and Humidity Stress Test
  • a semiconductor device for high-density mounting is required to operate normally even after 300 hours or more have elapsed in HAST, assuming an operating environment.
  • the Ag bonding wire has a problem in the life of the ball joint in HAST.
  • Spring failure is a failure that often becomes a problem in the connection of laminated chips used in memory applications in the high-density mounting field.
  • a connection method called reverse bonding in which the bonding position is reversed from that of normal bonding is often used.
  • a stud bump is formed on an electrode on a chip, a ball portion is bonded on an electrode on a substrate, and finally a bonding wire is wedge-bonded on the stud bump.
  • Chip damage is a defect that occurs in the ball bonding process in the bonding process.
  • a structure with low strength is often taken as the chip is thinned and multilayered, and the demand for chip damage suppression is increasing.
  • stress is concentrated at the time of bonding and chip damage is likely to occur, so that severe chip damage suppression is required.
  • An object of the present invention is to provide a bonding wire that can solve the problems of the prior art and satisfy the bonding reliability, spring performance, and chip damage performance required for high-density mounting.
  • the bonding wire for a semiconductor device includes one or more of In, Ga and Cd in a total amount of 0.05 to 5 at. %, And the balance consists of Ag and inevitable impurities.
  • the bonding wire according to the present embodiment is a total of 0.05 to 5 at. %, With the balance being Ag and inevitable impurities, it is possible to improve the bonding reliability required for high-density mounting and suppress spring failure.
  • the bonding wire according to the present embodiment When the bonding wire according to the present embodiment was used, an excellent ball joint life of 300 hours or more was obtained. From this result, the bonding wire according to the present embodiment satisfies the standard required for high-density mounting and can be used for high-density mounting. As a bonding condition when performing ball bonding to a semiconductor element using a fragile low dielectric constant material, it is necessary to make ultrasonic waves weaker than normal conditions in order to reduce damage to the semiconductor element. When such joining conditions are used, it is difficult to obtain a sufficient joining area with the conventional Ag bonding wire, and the ball joint life is less than 100 hours.
  • the effectiveness of the bonding wire according to this embodiment for suppressing spring failure will be described.
  • a spring failure occurred in the conventional Ag bonding wire.
  • the bonding wire according to the present embodiment the spring failure could be suppressed.
  • the bonding wire according to the present embodiment has confirmed the effect of suppressing the spring failure even in the most advanced high-density package.
  • the improvement effect of the spring performance is considered to be due to the improved yield strength of the bonding wire.
  • the bonding wire according to the present embodiment can satisfy the performance and cost reduction required for high-density mounting at the same time, and can replace the Au bonding wire.
  • a bonding wire containing more than% is not suitable for practical use because the breaking elongation of the bonding wire is lowered and the bonding wire is broken during wedge bonding. That is, in order to satisfy the overall performance as a bonding wire while improving the bonding reliability and suppressing the spring failure, the total of one or more of In, Ga and Cd contained in the bonding wire is 0.05 at. % To 5 at. % Is effective.
  • the bonding wire has a concentration of 0.1 to 2 at. % Is preferable because a life of 500 hours can be achieved in HAST.
  • the bonding wire has a concentration of 0.5 to 1 at. % Is more preferable because a life of 1000 hours can be achieved in HAST.
  • the shape called the tail at the time of wedge bonding is stabilized, variation in the shape and size when forming the ball can be reduced, and bonding reliability can be reduced. This is because variations can be suppressed.
  • an ICP emission spectroscopic analyzer or the like can be used for the concentration analysis of elements contained in the bonding wire.
  • the concentration measurement may be performed after scraping a 2 nm region from the surface by sputtering or the like before the analysis.
  • a method using pickling is also effective. The same method can be used for concentration analysis of Ni, Cu, Rh, Pd, Pt, Au or Be, B, P, Ca, Y, La, and Ce, which will be described later.
  • the bonding wire having the above characteristics further includes one or more of Ni, Cu, Rh, Pd, Pt, and Au in a total of 0.01 to 5 at. %, The service life of the bonding wire can be improved.
  • the bonding wire contains In, Ga, Cd, and further these elements and elements having a strong bonding force are added in combination.
  • the bonding wire according to the present embodiment has a total of 0.01 to 5 at. At least one of Ni, Cu, Rh, Pd, Pt, and Au. It has been found that the inclusion of at least 5% improves sulfidation resistance and can improve the service life of the bonding wire.
  • the concentration is 0.01 at. If it is less than%, the above effect cannot be expected.
  • the concentration is 5 at. If it exceeds%, heat input by arc discharge to the surface of the wire becomes unstable, and a ball with high sphericity cannot be obtained, which is not suitable for practical use.
  • the concentration is 0.5-3 at. If it is%, a higher effect can be obtained. This is because variation in heat input due to arc discharge can be further suppressed.
  • the bonding wire according to this embodiment further includes one or more of Be, B, P, Ca, Y, La, and Ce in a total of 10 to 300 at. By including ppm, the collapsed shape at the time of ball joining can be improved.
  • the ball In the conventional Ag bonding wire, the ball is preferentially deformed in the direction of application of the ultrasonic wave at the time of ball bonding, so that it may contact an adjacent electrode and cause a short circuit. Therefore, it is necessary to reduce the anisotropy of the ball deformation in the ball bonding and to control the collapsed shape close to a perfect circle. Since the anisotropy of ball deformation tends to increase as the crystal grain size increases, a technique for refining the crystal grains in the ball portion is effective.
  • the bonding wire according to the present embodiment includes one or more of Be, B, P, Ca, Y, La, and Ce in a total of 10 to 300 at. It has been found that the inclusion of ppm can make ball crystal grains finer and improve the collapsed shape during ball bonding.
  • the concentration is 10 at. In the case of less than ppm, the above effect cannot be expected.
  • the concentration is 300 at. If it exceeds ppm, the breaking elongation of the bonding wire is reduced, and the wire is broken during wedge bonding, which is not suitable for practical use.
  • the concentration is 20 to 200 at. If it is ppm, a higher effect can be obtained. This is because the concentration of elements in the ball can be reduced within this concentration range, and the elements are more uniformly distributed.
  • the bonding wire according to the present embodiment is a total of In.Ga, Cd at the surface of the bonding wire at. % Concentration is the total at. Wedge bondability can be improved by being at least twice the% concentration.
  • an Auger electron spectroscopy analyzer can be used for the concentration analysis in the depth direction from the bonding wire surface.
  • concentration measurement is performed while cutting the surface of the bonding wire by sputtering or the like, and a concentration profile in the depth direction is acquired.
  • the elements for which the concentration profile is acquired are Ag, In, Ga, Cd, and O.
  • the concentration of each element in each region is determined by obtaining the average concentration of each region by dividing into 0 to 10 nm region and 20 to 30 nm region from the wire surface in the depth direction.
  • the bonding wire In wedge bonding, the bonding wire is deformed to ensure a bonding area. Therefore, the softer the surface of the bonding wire, the easier it is to secure the bonding area and high bonding strength. Therefore, a technique for concentrating an element softer than Ag in the surface portion of the bonding wire with respect to the inside of the bonding wire is effective.
  • the inside of the bonding wire is defined as a region of 20 to 30 nm in the depth direction from the wire surface, and the surface portion of the bonding wire is defined as a region of 0 to 10 nm in the depth direction from the wire surface.
  • the present inventors have determined that the total amount of In, Ga, Cd on the surface of the bonding wire is at. % Is the total at. It has been found that a high bonding strength can be obtained at the wedge bonded portion by being at least twice the%. That is, the bonding wire, In to the total metal elements in the region of 0 ⁇ 10 nm in the depth direction from the wire surface, Ga, an average concentration of at least one element selected from the Cd X 0-10nm, When the average concentration in the region of 20 to 30 nm is X 20-30 nm, a high bonding strength can be obtained at the wedge bonding portion when X 0-10 nm / X 20-30 nm ⁇ 2 holds. When X 0-10 nm / X 20-30 nm ⁇ 2, the above effect cannot be expected.
  • the average crystal grain in the cross section perpendicular to the wire axis is 0.2 to 3.5 ⁇ m, so that the feeding property of the wire can be improved.
  • the wire axis is an axis that passes through the center of the bonding wire and is parallel to the longitudinal direction.
  • a crystal grain boundary can be determined by obtaining a crystal orientation difference between adjacent measurement points.
  • a crystal grain boundary having an orientation difference of 15 degrees or more is defined as a large tilt grain boundary, and a region surrounded by the large tilt grain boundary is defined as one crystal grain.
  • the crystal grain size was the diameter when the area was calculated with dedicated analysis software and the area was assumed to be a circle.
  • the bonding wires When joining bonding wires, the bonding wires are drawn out from a state where they are wound around a cylindrical jig called a spool. When drawing out, tension is applied to the bonding wire in the direction parallel to the wire axis, so that the bonding wire may be deformed and the wire diameter may be reduced. In order to prevent such a phenomenon, it is necessary to control the strength against the shear stress acting in the direction perpendicular to the wire axis. As a method for controlling the strength against shear stress, it is effective to reduce the crystal grain size in the cross section perpendicular to the wire axis.
  • the present inventors have found that high feeding performance can be obtained when the average crystal grain in the cross section perpendicular to the wire axis of the bonding wire is 0.2 to 3.5 ⁇ m. More preferably, a higher effect is obtained when the average particle diameter is 0.4 to 3.0 ⁇ m.
  • the average crystal grain size exceeds 3.5 ⁇ m, the wire is locally deformed by tensile stress, and thus the above-described effect cannot be obtained. If the average crystal grain size is less than 0.2 ⁇ m, the bonding wire is hardened more than necessary, and wear at the contact portion with the capillary becomes severe, which is not suitable for practical use.
  • the abundance ratio of the crystal orientation ⁇ 100> having an angle difference of 15 degrees or less with respect to the longitudinal direction of the bonding wire Is an area ratio of 30% or more and 100% or less, so that wedge bondability can be further improved.
  • the deformation of the joint can be promoted by increasing the abundance ratio of the crystal orientation ⁇ 100> in which the angle difference with respect to the longitudinal direction of the bonding wire is 15 degrees or less in the cross section of the bonding wire, and the high bonding strength Is obtained.
  • the area of the crystal orientation ⁇ 100> having an angle difference with respect to the longitudinal direction of the wire of 15 degrees or less may occupy 30% or more of the total area of the crystal orientation measurement area. It is valid.
  • the abundance ratio is less than 30%, deformation of the joint becomes insufficient, and high joint strength cannot be obtained at the wedge joint.
  • the crystal orientation of the cross section of the bonding wire can be determined using the EBSD method.
  • the existence ratio of the crystal orientation ⁇ 100> whose angle difference with respect to the longitudinal direction of the bonding wire is 15 degrees or less occupies the region having the crystal orientation ⁇ 100> with respect to the area of the crystal orientation measurement region using EBSD or the like. It can be determined by calculating the area ratio.
  • the measurement region has a cross section parallel to the wire axis including the wire axis, and the longitudinal direction of the wire is 100 ⁇ m or less, and the short side direction is the entire wire (approximately the same length as the wire diameter).
  • the present inventors have selected from In, Ga, and Cd with respect to the total amount of metal elements in the region of 0 to 1 nm in the depth direction from the outermost surface of the bonding wire according to the present embodiment. Average total of at least one element at. % In the region of 1 to 10 nm with respect to the depth direction from the outermost surface of the bonding wire. It has been found that the service life of the capillary can be improved by controlling to 1.2 times or more of%.
  • the bonding wire has an average concentration of at least one element selected from In, Ga and Cd with respect to the total amount of metal elements in the region of 0 to 1 nm in the depth direction from the wire surface, X 0-1 nm , If the average concentration in the region of 1 to 10 nm is X 1-10 nm , an excellent service life of the capillary can be obtained when X 0-1 nm / X 1-10 nm ⁇ 1.2 holds. When X 0-1 nm / X 1-10 nm ⁇ 1.2, the above effect cannot be expected.
  • Ag as a raw material has a purity of 99.9 at. % Or more and the balance is composed of inevitable impurities.
  • Ga, Cd, Ni, Cu, Rh, Pd, Pt, Au, Be, B, P, Ca, Y, La, and Ce have a purity of 99.9 at. % Or more and the balance is composed of inevitable impurities.
  • the Ag alloy used for the bonding wire is charged with a raw material in a carbon crucible processed into a cylindrical shape with a diameter of 3 to 6 mm, and 1080 to 1600 in a vacuum or an inert atmosphere such as N 2 or Ar gas using a high frequency furnace. It was manufactured by heating to 0 ° C. and dissolving, followed by furnace cooling or air cooling.
  • the obtained Ag alloy was drawn to ⁇ 0.9 to 1.2 mm, and then continuously drawn using a die to produce a ⁇ 300 to 600 ⁇ m wire. .
  • pickling treatment using hydrochloric acid or the like may be performed.
  • an intermediate heat treatment at 200 to 500 ° C. and a wire drawing process were repeated to obtain a final wire diameter of ⁇ 15 to 25 ⁇ m.
  • a commercially available lubricant was used for wire drawing, and the wire feed speed during wire drawing was 20 to 300 m / min.
  • the intermediate heat treatment was performed in an Ar gas atmosphere while continuously sweeping the wire.
  • the wire feed speed during the intermediate heat treatment was 20 to 200 m / min.
  • the intermediate heat treatment temperature is 200 to 330 ° C. for the first time, 250 to 400 ° C. for the second time, and 350 to 500 ° C. for the third and subsequent times. This is due to the effect that the element added by the heat treatment diffuses to the surface of the bonding wire.
  • the average crystal grain size in the cross section perpendicular to the wire axis can be controlled to 0.2 to 3.5 ⁇ m. This is due to the effect of controlling crystal grain growth during recrystallization.
  • the wire feed speed at the time of wire drawing to 200 to 300 m / min and the intermediate heat treatment temperature to 200 to 300 ° C.
  • the crystal orientation ⁇ 100> has an angle difference of 15 degrees or less with respect to the longitudinal direction of the bonding wire. The abundance ratio can be increased to 30% or more.
  • this technique is effective also when performing intermediate heat processing in multiple times.
  • the wire after the wire drawing was subjected to final heat treatment so that the final elongation at break was about 9-15%.
  • the final heat treatment was performed in the same manner as the intermediate heat treatment.
  • the wire feed rate during the final heat treatment was set to 20 to 200 m / min as in the intermediate heat treatment.
  • the final heat treatment temperature was 200 to 600 ° C., and the heat treatment time was 0.2 to 1.0 seconds.
  • an additional heat treatment is performed at 350 to 500 ° C. for 0.2 to 0.5 seconds, whereby the In region in the 0 to 1 nm region is compared with the 1 to 10 nm region in the depth direction from the wire surface. It is possible to control the concentration of at least one of Ga, Cd and Cd to 1.2 times or more.
  • the sample for bonding reliability evaluation is a commercially available epoxy resin by ball bonding using a commercially available wire bonder to an electrode in which a 1.0 ⁇ m thick Al film is formed on a Si substrate on a general metal frame. It was made by sealing with. The balls were formed while N 2 + 5% H 2 gas was flowed at a flow rate of 0.4 to 0.6 L / min, and the ball diameter was 1.5 to 1.6 times the wire diameter.
  • the bonding reliability in a high-temperature and high-humidity environment was determined by the bonding life of the ball joint when exposed to a high-temperature and high-humidity environment at a temperature of 130 ° C. and a relative humidity of 85% using an unsaturated pressure cooker tester.
  • the joint life of the ball joint was a time at which the shear test of the ball joint was conducted every 100 hours and the value of the shear strength was 1/3 of the shear strength obtained in the initial stage.
  • the shear test after the high-temperature and high-humidity test was conducted after removing the resin by acid treatment and exposing the ball joint.
  • the shear tester used was a micro strength tester manufactured by DAGE. As the value of the shear strength, an average value of 10 measured values of randomly selected ball joints was used.
  • a sample for spring performance evaluation was produced by performing reverse bonding, which is a bonding method for performing wedge bonding, on a stud bump formed on an electrode on a semiconductor element, using a commercially available wire bonder.
  • the joining conditions were a loop length of 3.0 mm and a loop height of 0.13 mm.
  • the loop portion of the 200 bonded bonding wires was observed with an optical microscope, and if there was a portion where the adjacent bonding wires contacted, it was determined to be defective. If there are 5 or more defects, it is judged that there is a practical problem. If it is 1 to 4 defects, it is judged that there is no practical problem. Judgment was made and marked as ⁇ .
  • ball bonding is performed using a commercially available wire bonder on an electrode in which an Al film having a thickness of 1.0 ⁇ m is formed on a Si substrate, and the Si substrate directly under the ball bonding portion is observed with an optical microscope.
  • a crack was seen in the Si substrate, it was determined to be defective.
  • 100 points were observed, it was judged that there was a practical problem if there were one or more defects.
  • the service life of the bonding wire is evaluated by allowing the bonding wire to stand in the air for a certain period of time, and then performing bonding to form a good ball or to obtain a good bonding state at the ball bonding portion and the wedge bonding portion. Evaluated whether or not.
  • the ball formation was judged as defective when 100 balls were observed with an optical microscope and there were 5 or more balls with low sphericity or irregularities on the surface.
  • the ball was formed using N 2 + 5% H 2 gas at a gas flow rate of 0.4 to 0.6 L / min, and the ball diameter was 1.5 to 1.6 times the wire diameter.
  • Whether or not a good bonding state was obtained at the ball bonded portion and the wedge bonded portion was determined by continuously performing bonding 1000 times using a commercially available wire bonder. Ball joints and wedge joints were observed with an optical microscope, and when three or more defects such as peeling occurred, it was determined to be defective. If any of the above defects occurs when the leaving period is less than 12 months, it is judged that there is a problem in practice, and a defect occurs when the leaving period is less than 18 months after 12 months Judged that there is no problem in practical use, ⁇ mark, if a defect occurs within less than 24 months after the leaving period of 18 months, ⁇ mark, after 24 months of leaving period If no defect occurred at all, it was judged to be particularly excellent and marked with a ⁇ mark.
  • Evaluation of the collapsed shape of the ball was performed by ball bonding using a commercially available wire bonder to an electrode in which an Al film having a thickness of 1.0 ⁇ m was formed on a Si substrate and observed with an optical microscope from directly above.
  • the collapsed shape of the ball was determined to be good if the collapsed shape was close to a circle, and was determined to be defective if it was an elliptical or petal-shaped shape. Observe 100 ball joints with an optical microscope. If there are 5 or more defects, it is judged that there is a practical problem. If there are 1 to 4 defects, it is judged that there is no practical problem. When no defect occurred at all, it was judged that the product was particularly excellent and marked with “ ⁇ ”.
  • Evaluation of wedge bondability was performed by performing wedge bonding using a commercially available wire bonder using a general metal frame subjected to Ag plating, and observing the wedge bonded portion.
  • the joining conditions used were generally used joining conditions. Observe 50 wedge joints with an optical microscope. If there are 5 or more bonding wires peeled off at the joint, it is judged that there is a practical problem. If there are 3 or 4 peelings, there is a practical problem. If it was judged that there was no defect, it was judged that it was excellent if it had 1 or 2 peels, and it was marked excellent if it did not cause any defects.
  • the bonding wire feeding performance is evaluated by observing the bonding wire in the loop part with a scanning microscope after measuring the bonding wire under general bonding conditions, measuring the diameter, and reducing the diameter of the bonding wire before bonding. I went by asking for. If the reduction rate was 80% or less, it was determined to be defective. Observe 30 bonding wires. If there are 5 or more defects, it is judged that there is a practical problem. If there are 3 or 4 defects, it is judged that there is no practical problem. It was judged that it was excellent if it was ⁇ 2 and marked as ⁇ , and if no defect occurred at all, it was judged as excellent and marked as ⁇ .
  • the service life of the capillary was evaluated by observing the hole at the tip of the capillary before and after use and the amount of wear of the hole at the tip of the capillary.
  • the bonding condition is a general condition. Observe the capillary after bonding the bonding wire 3000 times. If there is no wear, it is judged that there is no practical problem. If not, it was judged to be excellent and marked as ⁇ .
  • Table 1-1 to Table 1-10 show examples in which characteristics such as the composition of the bonding wire according to the present invention and evaluation results of each bonding wire are described.
  • Tables 2-1 and 2-2 show comparative examples.
  • the bonding wires according to claim 1 are No. 1 to 188, and it was confirmed that the bonding reliability, spring performance, and chip damage performance required for high-density mounting can be satisfied. In contrast, as shown in Comparative Examples Nos. 1 to 7, it was confirmed that sufficient effects could not be obtained when the In, Ga, and Cd concentrations were outside the above ranges.
  • the bonding wires according to claim 2 are Nos. 31 to 94, 123 to 127, 131 to 135, 140 to 144, 148 to 152, 156 to 160, 164 to 168, 173 to 177, 180, and 182; It was confirmed that the service life of can be improved.
  • the bonding wires according to claim 3 are Nos.
  • the bonding wires according to claim 4 are No. 1 to 127 and 136 to 180, and it was confirmed that good wedge bondability was obtained.
  • the bonding wires according to claim 5 were No. 1 to 135, 137 to 168, and 170 to 188, and it was confirmed that excellent wire feeding performance was obtained.
  • the bonding wires according to claim 6 are No. 170 to 188, and it was confirmed that superior wedge bondability was obtained.
  • the bonding wires according to claim 7 are Nos. 182 to 188, and it was confirmed that an excellent service life of the capillary was obtained.

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Abstract

　高密度実装で要求される接合信頼性、スプリング性能、チップダメージ性能を満足することができるボンディングワイヤを提供する。ボンディングワイヤは、Ｉｎ，Ｇａ，Ｃｄの１種以上を総計で０．０５～５ａｔ．％含み、残部がＡｇおよび不可避不純物からなることを特徴とする。

Description

半導体装置用ボンディングワイヤ
　本発明は、半導体素子上の電極と外部リード等の回路配線基板の配線を接続するために利用される半導体装置用ボンディングワイヤに関するものである。
　現在、半導体素子上の電極と外部リードの間を接合する半導体装置用ボンディングワイヤ（以下、ボンディングワイヤという）として、線径１５～５０μｍ程度の細線が主として使用されている。ボンディングワイヤの接合方法は超音波併用熱圧着方式が一般的であり、汎用ボンディング装置、ボンディングワイヤをその内部に通して接続するキャピラリ冶具等が用いられる。ボンディングワイヤの接合プロセスは、ワイヤ先端をアーク入熱で加熱溶融し、表面張力によりボールを形成した後に、１５０～３００℃の範囲内で加熱した半導体素子の電極上にこのボール部を圧着接合（以下、ボール接合という）し、次にループを形成した後、外部リード側の電極にワイヤ部を圧着接合（以下、ウェッジ接合という）することで完了する。ボンディングワイヤの接合相手である半導体素子上の電極には、Ｓｉ基板上にＡｌを主体とする合金膜を成膜した電極構造、外部リード側の電極にはＡｇめっき、Ｐｄめっきなどを施した電極構造が用いられることが多い。
　ボンディングワイヤには、優れたボール形成性、ボール接合性、ウェッジ接合性、ループ形成性などが要求される。これらの要求性能を総合的に満足するボンディングワイヤの材料としてＡｕが主に用いられてきた。近年では、Ａｕ価格の高騰を背景にＡｕの代替として比較的安価な材料を用いたボンディングワイヤの開発が盛んに行われている。開発例としてＣｕの表面にＰｄを被覆した構造を有するボンディングワイヤが挙げられる。このボンディングワイヤの特徴は、主にＣｕの酸化を抑制することでボンディングワイヤの性能を総合的に改善した点であり、最先端のＬＳＩ（Ｌａｒｇｅ　Ｓｃａｌｅ　Ｉｎｔｅｇｒａｔｉｏｎ）分野において使用されている。
　今後のボンディングワイヤ開発では、半導体デバイスの更なる高性能化、小型化に伴う高密度実装化への対応が強く求められている。高密度実装では、ＬＳＩ層間の信号遅延を抑制するために、層間の絶縁材料として脆弱な低誘電率材料が用いられることがあり、半導体素子へのダメージが問題となることが多い。隣接する電極の間隔が狭く、ボンディングワイヤの線径を細くする必要があるため、ボンディングワイヤは高いウェッジ接合性が要求される。細い線径において電気導電性を確保するためには、ボンディングワイヤに用いる材料の比抵抗は低い方が望ましい。このような高密度分野におけるボンディングワイヤの材料としては、軟質かつ高いウェッジ接合性が得られ、比較的比抵抗の低いＡｕが使用されることが多い。
　上記のような高密度実装化における課題を解決し、Ａｕに比べて安価なボンディングワイヤを提供するため、ボンディングワイヤの材料にＡｇを用いる試みがなされている。Ａｇのヤング率（約８３×１０９　Ｎ／ｍ２）は、Ａｕのヤング率（約８０×１０９Ｎ／ｍ２）とほぼ等しく、Ｃｕのヤング率（約１３０×１０９Ｎ／ｍ２）に比べると低いため、脆弱な半導体素子に対するボール接合において損傷が少なく、良好なウェッジ接合性が得られると期待される。室温付近におけるＡｇの比抵抗（１．６μΩ・ｃｍ）は、Ｃｕの比抵抗（１．７μΩ・ｃｍ）やＡｕの比抵抗（２．２μΩ・ｃｍ）に比べて低いことから、電気的特性の観点からも高密度実装におけるボンディングワイヤの材料として好適と考えられる。
　しかしながら、Ａｇを用いたボンディングワイヤ（以下、Ａｇボンディングワイヤという）は、高密度実装において接合信頼性やループの安定性が低いという課題がある。接合信頼性評価は、実際の半導体デバイスの使用環境における接合部寿命を評価する目的で行われる。一般的に接合信頼性評価には高温放置試験、高温高湿試験が用いられる。Ａｇボンディングワイヤは、Ａｕを用いたボンディングワイヤ（以下、Ａｕボンディングワイヤという）に比べて、高温高湿試験におけるボール接合部の寿命が劣ることが課題である。高密度実装では、小ボール接合が行われることから接合に寄与する面積が小さくなるため寿命を確保することがより一層困難となる。
　ループの安定性については、スプリング不良と呼ばれる不良が問題となる。スプリング不良は、ボンディングワイヤの接合工程でループが湾曲する現象でボンディングワイヤ同士が接触して短絡を引起こす原因となる。高密度実装では、隣接するボンディングワイヤの間隔が狭くなることから、スプリング不良の抑制が強く求められる。スプリング不良は、ワイヤの強度が低いほど発生し易くなることから、線径が細くなる高密度実装では問題となることが多い。
　これらの課題を解決する方法としてＡｇに種々の元素を添加し、合金化する技術が開示されているが、合金元素の濃度が高くなるとボールが硬くなりボール接合の際にチップダメージが発生する。これらの課題がＡｇボンディングワイヤの普及を妨げている原因となっている。
　接合信頼性の改善を目的としたＡｇボンディングワイヤの開発については、例えば、特許文献１に、Ｐｔ、Ｐｄ、Ｃｕ、Ｒｕ、Ｏｓ、Ｒｈ、Ｉｒの１種または２種以上を合計で０．１～１０重量％含み、Ｐｔが１０重量％以下、Ｐｄが１０重量％以下、Ｃｕが５重量％以下、Ｒｕが１重量％以下、Ｏｓが１重量％以下、Ｒｈが１重量％以下、Ｉｒが１重量％以下であり、残部がＡｇ及び不可避不純物からなることを特徴とするボンディングワイヤが開示されている。
　例えば、特許文献２に、純度９９．９９質量％以上のＡｇと純度９９．９９９質量％以上のＡｕと純度９９．９９質量％以上のＰｄとからなる三元合金系ボンディングワイヤであって、Ａｕが４～１０質量％、Ｐｄが２～５質量％、酸化性非貴金属添加元素が１５～７０質量ｐｐｍおよび残部がＡｇからなり、当該ボンディングワイヤは連続ダイス伸線前に焼きなまし熱処理がされ、連続ダイス伸線後に調質熱処理がされ、窒素雰囲気中でボールボンディングされることを特徴とする半導体素子用Ａｇ－Ａｕ－Ｐｄ三元合金系ボンディングワイヤが開示されている。
　ループの安定性向上を目的としたＡｇボンディングワイヤの開発については、加工熱処理によって引張強度と０．２％耐力を制御する技術が開示されている。例えば、特許文献３に、５２３Ｋの温度雰囲気にて１５～２５秒間加熱した後に引き続き前記温度雰囲気で測定した引張強度が、２９８Ｋの温度雰囲気にて測定した０．２％耐力よりも高いことを特徴とするボンディングワイヤが開示されている。
特開平１１－２８８９６２号公報 特開２０１２－１６９３７４号公報 特開２００２－３１９５９７号公報
　しかしながら上記特許文献に開示されているＡｇボンディングワイヤでは、高密度実装で要求される接合信頼性、スプリング不良、チップダメージに対する要求性能を満たすことができなかった。
　高温高湿試験は温度が１２１℃、相対湿度が１００％の条件で行うＰＣＴ（Ｐｒｅｓｓｕｒｅ　Ｃｏｏｋｅｒ　Ｔｅｓｔ）と呼ばれる試験が一般的に用いられる。近年では、さらに厳しい評価方法として温度が１３０℃、相対湿度が８５％の条件で行うＨＡＳＴ（Ｈｉｇｈｌｙ　Ａｃｃｅｌｅｒａｔｅｄ　ｔｅｍｐｅｒａｔｕｒｅ　ａｎｄ　ｈｕｍｉｄｉｔｙ　Ｓｔｒｅｓｓ　Ｔｅｓｔ）と呼ばれる試験が用いられることが多い。高密度実装用の半導体デバイスは、動作環境を想定した場合、ＨＡＳＴにおいて３００時間以上経過後も正常に動作することが求められる。Ａｇボンディングワイヤは、ＨＡＳＴにおいてボール接合部の寿命が問題となる。Ａｇボンディングワイヤは、高温高湿環境に曝されることで、ボール接合部において剥離が発生し、電気的な接続が失われて半導体デバイスの故障の原因となる。また、高密度実装では、狭ピッチ化に対応するため通常よりも小さなボールを形成して接合（以下、小ボール接合という）することが多い。ボンディングワイヤは、小ボール接合を用いた場合、接合に寄与する面積が小さくなり接合寿命が短くなる傾向にあることから、より厳しい接合信頼性が求められる。
　スプリング不良は高密度実装分野の中でもメモリ用途で行われる積層チップの接続で問題となることが多い不良である。積層チップをボンディングワイヤによって接続する方法では、通常の接合とは接合位置が逆転する、逆ボンディングと呼ばれる接続方法が多く用いられる。逆ボンディングの接合プロセスでは、チップ上の電極にスタッドバンプを形成した後、基板の電極上にボール部を接合し、最後に前記スタッドバンプの上にボンディングワイヤをウェッジ接合する。この逆ボンディングにより、ループ高さを低く抑えることができ、チップの積層数が増えて段差がかなり高い場合においても安定したループ制御が可能となる。一方、逆ボンディングを行うと、スプリング不良が発生しやすい。
　チップダメージは、ボンディング工程の中でもボール接合工程において発生する不良である。高密度実装分野ではチップの薄化、多層化に伴い強度が低い構造が取られることが多く、チップダメージ抑制に対する要求が高まっている。さらに、高密度実装で行われる小ボール接合では、接合の際に応力が集中してチップダメージが発生し易くなるため、チップダメージの厳しい抑制が求められる。
　これらの要求性能に対して、従来報告されているＡｇボンディングワイヤを用いた場合、接合信頼性、スプリング性能、チップダメージ性能において以下の課題があることが判明した。接合信頼性については、Ａｌ電極にボール接合を行い、樹脂封止後、ＨＡＳＴを行った結果、１５０時間経過した段階でボール接合部の接合強度が低下し、高密度実装で要求される３００時間以上の寿命が得られなかった。接合界面の観察を行った結果、ＡｇボンディングワイヤとＡｌ電極の界面において、ボイドの発生が認められた。これは接合界面に形成されたＡｇとＡｌの金属間化合物の一部が腐食したためと推定された。スプリング性能については、逆ボンディングを行った場合に、ボンディングワイヤの強度が不足してループが湾曲し、スプリング不良の抑制が困難であることが判明した。チップダメージ性能については、小ボール接合を行った場合において、チップにき裂が発生し実用に適さないことがわかった。
　本発明では、従来技術の問題を解決して高密度実装で要求される接合信頼性、スプリング性能、チップダメージ性能を満足することができるボンディングワイヤを提供することを目的とする。
　本発明の半導体装置用ボンディングワイヤは、Ｉｎ，Ｇａ，Ｃｄの１種以上を総計で０．０５～５ａｔ．％含み、残部がＡｇおよび不可避不純物からなることを特徴とする。
　本発明によれば、高密度実装で要求される接合信頼性、スプリング性能、チップダメージ性能を満足することができる。
　本実施形態に係るボンディングワイヤは、Ｉｎ，Ｇａ，Ｃｄの１種以上を総計で０．０５～５ａｔ．％含み、残部がＡｇおよび不可避不純物とすることで、高密度実装に要求される接合信頼性を改善し、スプリング不良を抑制することができる。
　本実施形態に係るボンディングワイヤの接合信頼性の改善に対する有効性を説明する。ワイヤ線径がφ１５～２５μｍのワイヤを用いた場合、通常の接合では、ワイヤ線径に対してボール径が１．７～２．０倍のボールを形成して接合が行われる。高密度実装用途では、狭ピッチ化に対応するため、ワイヤ線径に対してボール径が１．５～１．６倍と通常よりも小さなボールを形成して接合することが多い。従来のＡｇボンディングワイヤを用いて小ボール接合し、温度が１３０℃、相対湿度が８５％の条件で高温高湿試験を行った場合、ボール接合部寿命は１５０時間未満であった。本実施形態に係るボンディングワイヤを用いた場合、３００時間以上の優れたボール接合部寿命が得られた。この結果から、本実施形態に係るボンディングワイヤは、高密度実装において要求される基準を満たしており、高密度実装において使用することができる。脆弱な低誘電率材料を用いた半導体素子にボール接合を行う際の接合条件は、半導体素子の損傷を緩和するために、通常の条件よりも超音波を弱くする必要がある。このような接合条件を用いた場合、従来のＡｇボンディングワイヤでは、十分な接合面積を得ることが困難であり、ボール接合部寿命は１００時間未満であった。一方で、本実施形態に係るボンディングワイヤを用いた場合、３００時間以上のボール接合部寿命が得られ、優れた接合信頼性が実現できることが確認された。これは、本実施形態に係るボンディングワイヤに含まれるＩｎ，Ｇａ，Ｃｄがボール接合部の接合界面において、腐食の原因となるＡｇとＡｌの金属間化合物の成長を抑制したためと考えられる。
　次に、本実施形態に係るボンディングワイヤのスプリング不良抑制に対する有効性を説明する。複数の半導体素子が積層された構造を有するデバイスに対して、ワイヤ線径がφ１５～２５μｍのワイヤを用いて逆ボンディングを実施したところ、従来のＡｇボンディングワイヤではスプリング不良が発生した。これに対して、本実施形態に係るボンディングワイヤではスプリング不良が抑制できた。本実施形態に係るボンディングワイヤは、最先端の高密度パッケージにおいてもスプリング不良の抑制効果が確認できた。スプリング性能の改善効果は、ボンディングワイヤの降伏強度が向上したことによるものと考えられる。
　以上より、本実施形態に係るボンディングワイヤは、高密度実装で要求される性能と抵コスト化を同時に満足でき、Ａｕボンディングワイヤの代替が可能であることが明らかになった。
　Ｉｎ，Ｇａ，Ｃｄの１種以上を総計で５ａｔ．％超含むボンディングワイヤは、ボンディングワイヤの破断伸びが低下し、ウェッジ接合時にボンディングワイヤが切れる不良が発生するため実用に適さない。すなわち、接合信頼性を改善及びスプリング不良を抑制しつつボンディングワイヤとしての総合性能を満足するため、ボンディングワイヤに含まれるＩｎ，Ｇａ，Ｃｄの１種以上の総計は、０．０５ａｔ．％～５ａｔ．％であることが有効である。ボンディングワイヤは、前記濃度が０．１～２ａｔ．％であれば、ＨＡＳＴにおいて、５００時間の寿命が達成できるので、好ましい。これは、ボールの硬さを適正な範囲内に制御することで、ボール接合界面の金属間化合物が均一に形成されて、接合寿命低下の原因となる凹凸に起因する応力集中を緩和できるためである。さらにボンディングワイヤは、前記濃度が０．５～１ａｔ．％であれば、ＨＡＳＴにおいて、１０００時間の寿命が達成できるので、より好ましい。これは、ワイヤ部分の硬さを適正な範囲内に制御することでウェッジ接合時のテールと呼ばれる形状が安定し、ボールを形成する際の形状や大きさのばらつきが低減でき、接合信頼性のばらつきを抑制できるためである。
　ボンディングワイヤに含まれる元素の濃度分析には、ＩＣＰ発光分光分析装置等を利用することができる。ボンディングワイヤの表面に酸素や炭素などの元素が吸着している場合には、解析を行う前に表面から２ｎｍの領域をスパッタ等で削ってから濃度測定を行っても良い。その他の方法として、酸洗を用いる方法も有効である。後述するＮｉ，Ｃｕ，Ｒｈ，Ｐｄ，Ｐｔ，ＡｕまたはＢｅ，Ｂ，Ｐ，Ｃａ，Ｙ，Ｌａ，Ｃｅの濃度分析についても同様の方法を用いることができる。
　上記の特徴を有するボンディングワイヤは、さらにＮｉ，Ｃｕ，Ｒｈ，Ｐｄ，Ｐｔ，Ａｕの１種以上を総計で０．０１～５ａｔ．％含むことでボンディングワイヤの使用寿命を改善できる。
　ボンディングワイヤは、Ｉｎ，Ｇａ，Ｃｄを含み、更にこれらの元素と結合力が強い元素を複合添加することが、経時劣化に対して有効である。
　従来のボンディングワイヤは、時間の経過にともなって表面に硫黄原子が吸着し、ボール形成性などの性能が低下することがあった。ボンディングワイヤ表面の硫黄原子の吸着を抑制するためには、ボンディングワイヤ表面の活性を低下させる手法が有効である。例えば、ボンディングワイヤ表面のＡｇ原子をＡｇに比べて硫黄との吸着能が低い元素で置換すれば良い。ボンディングワイヤの表面にはＩｎ，Ｇａ，Ｃｄが存在することから、これらの元素と結合力の強い元素を添加することでより効率的に耐硫化性が向上できると考えられる。
　本発明者らは、鋭意検討した結果、本実施形態に係るボンディングワイヤが、Ｎｉ，Ｃｕ，Ｒｈ，Ｐｄ，Ｐｔ，Ａｕの１種以上を総計で０．０１～５ａｔ．％以上含むことで耐硫化性が向上し、ボンディングワイヤの使用寿命を改善できることを見出した。前記濃度が０．０１ａｔ．％未満の場合は上記の効果が期待できない。前記濃度が５ａｔ．％超の場合は、ワイヤ表面へのアーク放電による入熱が不安定になり、真球性の高いボールが得られなくなるため実用に適さない。好ましくは、前記濃度が０．５～３ａｔ．％であればより高い効果が得られる。これは、アーク放電による入熱のばらつきを、より抑制できるためである。
　本実施形態に係るボンディングワイヤは、さらにＢｅ，Ｂ，Ｐ，Ｃａ，Ｙ，Ｌａ，Ｃｅの１種以上を総計で１０～３００ａｔ．ｐｐｍ含むことで、ボール接合時のつぶれ形状を改善できる。
　従来のＡｇボンディングワイヤは、ボール接合時にボールが超音波の印加方向に優先的に変形するため、隣接する電極に接触し短絡を引起こすことがあった。したがって、ボール接合においてボール変形の異方性を低減し、真円に近いつぶれ形状に制御する必要がある。ボール変形の異方性は結晶粒径が大きいほど増加する傾向があるため、ボール部の結晶粒を微細化する技術が有効である。
　本発明者らは、鋭意検討した結果、本実施形態に係るボンディングワイヤが、Ｂｅ，Ｂ，Ｐ，Ｃａ，Ｙ，Ｌａ，Ｃｅの１種以上を総計で１０～３００ａｔ．ｐｐｍ含むことでボール結晶粒が微細化でき、ボール接合時のつぶれ形状が改善できることを見出した。前記濃度が１０ａｔ．ｐｐｍ未満の場合は上記の効果が期待できない。前記濃度が３００ａｔ．ｐｐｍ超の場合は、ボンディングワイヤの破断伸びが低下し、ウェッジ接合時にワイヤが切れてしまうため実用に適さない。好ましくは、前記濃度が２０～２００ａｔ．ｐｐｍであればより高い効果が得られる。これは、この濃度範囲であればボール中の元素の偏りを低減でき、元素がより均一に分布するためである。
　本実施形態に係るボンディングワイヤは、ボンディングワイヤ表面部のＩｎ，Ｇａ，Ｃｄの総計ａｔ．％濃度が、ボンディングワイヤ内部の前記総計ａｔ．％濃度の２倍以上であることで、ウェッジ接合性を改善できる。
　ボンディングワイヤ表面から深さ方向の濃度分析は、オージェ電子分光分析装置を用いることができる。まず、ボンディングワイヤの表面からスパッタ等で削りながら濃度測定を行い、深さ方向の濃度プロファイルを取得する。濃度プロファイルを取得する対象の元素はＡｇ、Ｉｎ，Ｇａ，Ｃｄ、Ｏとする。ワイヤ表面から深さ方向に対して０～１０ｎｍの領域、２０～３０ｎｍの領域に分けて、各領域の平均濃度を求めることでよって各領域におけるそれぞれの元素の濃度を決定する。
　ウェッジ接合では、ボンディングワイヤを変形させて接合面積を確保するため、ボンディングワイヤの表面部が軟質であるほど接合面積の確保が容易になり、高い接合強度が得られる。したがって、ボンディングワイヤの内部に対して、ボンディングワイヤの表面部にＡｇよりも軟質な元素を濃化させる技術が有効である。ボンディングワイヤの内部をワイヤ表面から深さ方向に２０～３０ｎｍの領域、ボンディングワイヤの表面部をワイヤ表面から深さ方向に０～１０ｎｍの領域と定義する。
　本発明者らは、鋭意検討した結果、ボンディングワイヤ表面部のＩｎ，Ｇａ，Ｃｄの総計ａｔ．％が、ボンディングワイヤ内部の前記総計ａｔ．％の２倍以上であることで、ウェッジ接合部において高い接合強度が得られることを見出した。すなわち、ボンディングワイヤは、ワイヤ表面から深さ方向に０～１０ｎｍの領域における金属元素の総計に対するＩｎ，Ｇａ，Ｃｄの中から選ばれた少なくとも１つ以上の元素の平均濃度をＸ０－１０ｎｍ、２０～３０ｎｍの領域における該平均濃度をＸ２０－３０ｎｍとすれば、Ｘ０－１０ｎｍ／Ｘ２０－３０ｎｍ≧２が成立する場合において、ウェッジ接合部において高い接合強度が得られる。Ｘ０－１０ｎｍ／Ｘ２０－３０ｎｍ＜２の場合は、上記効果が期待できない。
　本実施形態に係るボンディングワイヤは、ワイヤ軸に垂直方向の断面における平均結晶粒が０．２～３．５μｍであることで、ワイヤの繰出し性が改善できる。ここでワイヤ軸とは、ボンディングワイヤの中心を通り、長手方向に平行な軸である。
　ワイヤ断面を露出させる方法は、例えば、機械研磨、イオンエッチング法等を利用することができる。平均結晶粒径を求める方法は、例えば後方散乱電子線回折法（ＥＢＳＤ：Ｅｌｅｃｔｒｏｎ　Ｂａｃｋｓｃａｔｔｅｒｅｄ　Ｄｉｆｆｒａｃｔｉｏｎ）を用いることができる。ＥＢＳＤ法は隣り合う測定点間の結晶方位差を求めることで、結晶粒界を判定することができる。結晶粒界は方位差が１５度以上のものを大傾角粒界と定義し、大傾角粒界に囲まれた領域を１つの結晶粒とした。結晶粒径は、専用の解析ソフトによって面積を算出し、その面積を円と仮定したときの直径とした。
　ボンディングワイヤを接合する際には、ボンディングワイヤをスプールと呼ばれる円柱状の冶具に巻取った状態から少量ずつ繰り出して使用する。繰り出しを行うときにはボンディングワイヤにはワイヤ軸と平行方向に張力がかかるため、ボンディングワイヤが変形して線径が細くなってしまう恐れがある。このような現象を防ぐためには、ワイヤ軸と垂直方向に働くせん断応力に対する強度を制御する必要がある。せん断応力に対する強度を制御する方法としては、ワイヤ軸と垂直方向の断面における結晶粒径を小さくすることが有効である。
　本発明者らは、鋭意検討した結果、ボンディングワイヤのワイヤ軸と垂直方向の断面における平均結晶粒が０．２～３．５μｍであることで高い繰り出し性能が得られることを見出した。さらに好ましくは、前記平均粒径が０．４～３．０μｍであればより高い効果が得られる。前記平均結晶粒径が３．５μｍ超では、引張応力によりワイヤが局部的に変形してしまうため、上記の効果が得られない。前記平均結晶粒径が０．２μｍ未満では、ボンディングワイヤが必要以上に硬質化してしまうためキャピラリとの接触部における摩耗が激しくなるため実用に適さない。
　本実施形態に係るボンディングワイヤは、ボンディングワイヤの断面の結晶方位を測定したときの測定結果において、前記ボンディングワイヤの長手方向に対して角度差が１５度以下である結晶方位＜１００＞の存在比率が面積率で、３０％以上１００％以下であることで、ウェッジ接合性をさらに改善できる。
　ウェッジ接合性に関しては、ボンディングワイヤの断面において、ボンディングワイヤの長手方向に対する角度差が１５度以下である結晶方位＜１００＞の存在比率を増加させることで接合部の変形が促進でき、高い接合強度が得られる。上記効果を得るためには、ワイヤの長手方向に対する角度差が１５度以下の結晶方位＜１００＞を有する領域の面積が、結晶方位の測定領域の総面積に対して３０％以上を占めることが有効である。前記存在比率が３０％未満では、接合部の変形が不十分となり、ウェッジ接合部において高い接合強度が得られない。
　ボンディングワイヤの断面を露出させる方法としては、機械研磨、イオンエッチング法等を利用することができる。ボンディングワイヤの断面の結晶方位はＥＢＳＤ法を用いて決定することができる。ボンディングワイヤの長手方向に対する角度差が１５度以下の結晶方位＜１００＞の存在比率は、ＥＢＳＤ等を用いた結晶方位の測定領域の面積に対して、前記結晶方位＜１００＞を有する領域が占める面積の比率を算出することによって求めることができる。前記測定領域は、ワイヤ軸を含むワイヤ軸に平行な断面であって、ワイヤの長手方向を１００μｍ以下、短手方向をワイヤ全体（ワイヤ直径と略同じ長さ）とする。
　本発明者らは、鋭意検討した結果、本実施形態に係るボンディングワイヤの最表面から深さ方向に対して０～１ｎｍの領域における金属元素の総計に対するＩｎ，Ｇａ，Ｃｄの中から選ばれた少なくとも１つ以上の元素の平均総計ａｔ．％が、ボンディングワイヤの最表面から深さ方向に対して１～１０ｎｍの領域における該平均総計ａｔ．％の１．２倍以上に制御することで、キャピラリの使用寿命が改善できることを見出した。
　キャピラリとボンディングワイヤが、ワイヤを繰り出す際の摩擦によって、キャピラリの内部が磨耗する課題があった。上記課題に対して、ボンディングワイヤの最表面の組成を制御し、ボンディングワイヤの最表面の強度を低減させることで、キャピラリとボンディングワイヤ間の摩擦力が低減でき、キャピラリの使用寿命を改善できる。すなわち、ボンディングワイヤは、ワイヤ表面から深さ方向に０～１ｎｍの領域における金属元素の総計に対するＩｎ，Ｇａ，Ｃｄの中から選ばれた少なくとも１つ以上の元素の平均濃度をＸ０－１ｎｍ、１～１０ｎｍの領域における該平均濃度をＸ１－１０ｎｍとすれば、Ｘ０－１ｎｍ／Ｘ１－１０ｎｍ≧１．２が成立する場合において、優れたキャピラリの使用寿命が得られる。Ｘ０－１ｎｍ／Ｘ１－１０ｎｍ＜１．２の場合は、上記効果が期待できない。
　以下、実施例について詳細に説明する。原材料となるＡｇは純度が９９．９ａｔ．％以上で、残部が不可避不純物から構成されるものを用いた。Ｉｎ，Ｇａ，Ｃｄ，Ｎｉ，Ｃｕ，Ｒｈ，Ｐｄ，Ｐｔ，Ａｕ，Ｂｅ，Ｂ，Ｐ，Ｃａ，Ｙ，Ｌａ，Ｃｅは、純度が９９．９ａｔ．％以上で残部が不可避不純物から構成されるものを用いた。
　ボンディングワイヤに用いるＡｇ合金は、直径がφ３～６ｍｍの円柱型に加工したカーボンるつぼに原料を装填し、高周波炉を用いて、真空中もしくはＮ２、Ａｒガス等の不活性雰囲気で１０８０～１６００℃まで加熱して溶解させた後、炉冷もしくは空冷を行うことで製造した。
　得られたＡｇ合金に対して、引抜加工を行ってφ０．９～１．２ｍｍまで加工した後、ダイスを用いて連続的に伸線加工等を行うことによって、φ３００～６００μｍのワイヤを作製した。このとき、ワイヤ表面に酸素や硫黄が吸着している場合には、塩酸等を用いた酸洗処理を行ってもよい。その後、２００～５００℃の中間熱処理と伸線加工を繰返し行うことによって最終線径のφ１５～２５μｍまで加工した。伸線には市販の潤滑液を用い、伸線時のワイヤ送り速度は２０～３００ｍ／分とした。中間熱処理はワイヤを連続的に掃引しながら、Ａｒガス雰囲気中で行った。中間熱処理時のワイヤの送り速度は２０～２００ｍ／分とした。ここで、２００～５００℃の中間熱処理の回数を３回以上行うことで、ワイヤ表面から深さ方向に２０～３０ｎｍの領域に対して、０～１０ｎｍの領域におけるＩｎ，Ｇａ，Ｃｄの１種以上の濃度を高く制御することが可能である。好ましくは中間熱処理温度が１回目は２００～３３０℃、２回目は２５０～４００℃、３回目以降は３５０℃～５００℃の範囲で行うことがより効果的である。これは上記の熱処理によって添加した元素がボンディングワイヤの表面に拡散する効果によるものである。また、中間熱処理を実施する線径をφ５０～φ１００μｍ以上とすることで、ワイヤ軸に垂直な方向の断面における平均結晶粒径を０．２～３．５μｍに制御できる。これは、再結晶時の結晶粒成長を制御できる効果によるものである。さらに、伸線時のワイヤ送り速度を２００～３００ｍ／分とし、中間熱処理の温度を２００～３００℃とすることでボンディングワイヤの長手方向に対する角度差が１５度以下である結晶方位＜１００＞の存在比率を３０％以上に増加させることができる。なお、本技術は中間熱処理を複数回行う場合においても有効である。
　伸線加工後のワイヤは最終的に破断伸びが約９～１５％になるよう最終熱処理を実施した。最終熱処理は中間熱処理と同様の方法で行った。最終熱処理時のワイヤの送り速度は中間熱処理と同様に２０～２００ｍ／分とした。最終熱処理温度は２００～６００℃で熱処理時間は０．２～１．０秒とした。ここで、最終熱処理後に追加熱処理を３５０～５００℃で０．２～０．５秒間実施することで、ワイヤ表面から深さ方向に１～１０ｎｍの領域に対して、０～１ｎｍの領域におけるＩｎ，Ｇａ，Ｃｄの１種以上の濃度を１．２倍以上に制御することが可能である。
　接合信頼性評価用のサンプルは、一般的な金属フレーム上のＳｉ基板に厚さ１．０μｍのＡｌ膜を成膜した電極に、市販のワイヤボンダーを用いてボール接合を行い、市販のエポキシ樹脂によって封止して作製した。ボールはＮ２＋５％Ｈ２ガスを流量０．４～０．６Ｌ／ｍｉｎで流しながら形成し、ボール径はワイヤ線径に対して１．５～１．６倍の範囲とした。高温高湿環境における接合信頼性は、不飽和型プレッシャークッカー試験機を使用し、温度１３０℃、相対湿度８５％の高温高湿環境に暴露した時のボール接合部の接合寿命によって判定した。ボール接合部の接合寿命は１００時間毎にボール接合部のシェア試験を実施し、シェア強度の値が初期に得られたシェア強度の１／３となる時間とした。高温高湿試験後のシェア試験は、酸処理によって樹脂を除去して、ボール接合部を露出させてから行った。シェア試験機はＤＡＧＥ社製の微小強度試験機を用いた。シェア強度の値は無作為に選択したボール接合部の１０か所の測定値の平均値を用いた。上記の評価において、接合寿命が３００時間未満であれば実用上問題があると判断し△印、３００以上５００時間未満であれば、実用上問題ないと判断し○印、５００時間以上であれば特に優れていると判断し◎印、１０００時間以上であれば☆印と表記した。
　スプリング性能評価用のサンプルは、半導体素子上の電極に形成したスタッドバンプ上に、市販のワイヤボンダーを用いて、ウェッジ接合を行う接合方法である逆ボンディングを行うことで作製した。接合条件は、ループ長３．０ｍｍ、ループ高さ０．１３ｍｍとした。接合した２００本のボンディングワイヤのループ部分を光学顕微鏡で観察し、隣接するボンディングワイヤが接触した箇所があれば不良と判定した。不良が５箇所以上あれば実用上問題があると判断し△印、不良が１～４箇所であれば実用上問題がないと判断し○印、不良が全く発生しなければ特に優れていると判断し◎印と表記した。
　チップダメージ性能の評価は、Ｓｉ基板に厚さ１．０μｍのＡｌ膜を成膜した電極に、市販のワイヤボンダーを用いてボール接合を行い、ボール接合部直下のＳｉ基板を光学顕微鏡で観察することによって行った。Ｓｉ基板に、き裂が見られた場合は不良と判定した。１００箇所観察し、不良が１箇所以上あれば実用上問題があると判断し△印、不良が全く発生しなければ特に優れていると判断し○印と表記した。
　ボンディングワイヤの使用寿命の評価は、ボンディングワイヤを大気雰囲気に一定期間放置した後、接合を行い、良好なボール形成ができているか、ボール接合部およびウェッジ接合部において良好な接合状態が得られているかどうかを評価した。ボール形成の判定は、１００個のボールを光学顕微鏡で観察し、真球性の低いボールや表面に凹凸のあるボールが５個以上あれば不良と判定した。ボールの形成条件は、Ｎ２＋５％Ｈ２ガスを使用してガス流量０．４～０．６Ｌ／ｍｉｎ、ボールの直径はワイヤ線径の１．５～１．６倍の範囲とした。ボール接合部およびウェッジ接合部において良好な接合状態が得られているかの判定は、市販のワイヤボンダーを用いて１０００回の接合を連続的に行って判定した。ボール接合部やウェッジ接合部を光学顕微鏡で観察し、剥離などの不良が３本以上発生した場合は不良と判定した。放置期間が１２か月未満で上記のいずれかの不良が発生した場合は実用上問題があると判断し△印、放置期間が１２か月経過後１８か月未満の間に不良が発生した場合は実用上問題がないと判断し○印、放置期間が１８か月経過後２４か月未満の間に不良が発生した場合は優れていると判断し◎印、放置期間が２４か月経過後も不良が全く発生しなければ特に優れていると判断し☆印と表記した。
　ボールのつぶれ形状の評価は、Ｓｉ基板に厚さ１．０μｍのＡｌ膜を成膜した電極に、市販のワイヤボンダーを用いてボール接合を行い、直上から光学顕微鏡で観察した。ボールのつぶれ形状の判定は、つぶれ形状が円形に近いのであれば良好と判定し、楕円形や花弁状の形状であれば不良と判定した。１００箇所のボール接合部を光学顕微鏡で観察し、不良が５個以上あれば実用上問題があると判断し△印、不良が１～４個であれば実用上問題がないと判断し○印、不良が全く発生しなければ特に優れていると判断し◎印と表記した。
　ウェッジ接合性の評価は、Ａｇめっきを施した一般的な金属フレームを用い、市販のワイヤボンダーを用いてウェッジ接合を行い、ウェッジ接合部を観察することで行った。接合条件は一般的に用いられる接合条件を用いた。５０本のウェッジ接合部を光学顕微鏡で観察し、接合部においてボンディングワイヤの剥離が５個以上あれば実用上問題があると判断し△印、剥離が３～４個であれば実用上問題がないと判断し○印、剥離が１～２個であれば優れていると判断し◎印、不良が全く発生しなければ特に優れていると判断し☆印と表記した。
　ボンディングワイヤの繰り出し性能の評価は、一般的な接合条件で接合を行った後、ループ部分のボンディングワイヤを走査型顕微鏡で観察し、直径を測定して、接合前のボンディングワイヤに対する直径の減少率を求めることで行った。減少率が８０％以下であれば不良と判定した。３０本のボンディングワイヤを観察し、不良が５本以上あれば実用上問題があると判断し△印、不良が３～４本であれば実用上問題がないと判断し○印、不良が１～２本であれば優れていると判断し◎印、不良が全く発生しなければ特に優れていると判断し☆印と表記した。
　キャピラリの使用寿命は、使用前後でキャピラリの先端の孔を観察し、キャピラリの先端の孔の磨耗量によって評価した。接合条件は一般的な条件とし、ボンディングワイヤを３０００回接合後のキャピラリを観察して、磨耗がなければ実用上問題ないと判断し○印、１００００回接合後のキャピラリを観察して、磨耗がなければ優れていると判断し◎印と表記した。
　表１－１～表１－１０は本発明に係わるボンディングワイヤの組成などの特徴とそれぞれのボンディングワイヤの各評価結果を記載した実施例を示している。表２－１及び表２－２は比較例を示している。
　請求項１に係るボンディングワイヤはNo.1～188であり、高密度実装で要求される接合信頼性とスプリング性能、チップダメージ性能を満足できることを確認した。これに対し比較例のNo.1～7に示すように、Ｉｎ，Ｇａ，Ｃｄの濃度が上記の範囲外の場合には十分な効果が得られないことを確認した。請求項２に係るボンディングワイヤはNo.31～94、123～127、131～135、140～144、148～152、156～160、164～168、173～177、180、182であり、ボンディングワイヤの使用寿命の向上が図れることを確認した。請求項３に係るボンディングワイヤはNo.95～127、134、135、143、144、151、152、159、160、167、168、176、177であり、優れたボールつぶれ形状が得られることを確認した。請求項４に係るボンディングワイヤはNo.1～127、136～180であり、良好なウェッジ接合性が得られることを確認した。請求項５に係るボンディングワイヤはNo.1～135、137～168、170～188であり、優れたワイヤ繰り出し性能が得られることを確認した。請求項６に係るボンディングワイヤはNo.170～188であり、より優れたウェッジ接合性が得られることを確認した。請求項７に係るボンディングワイヤはNo.182～188であり、優れたキャピラリの使用寿命が得られることを確認した。
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000008
Figure JPOXMLDOC01-appb-T000009
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Figure JPOXMLDOC01-appb-T000011
Figure JPOXMLDOC01-appb-T000012

Claims (7)

  1. Ｉｎ，Ｇａ，Ｃｄの１種以上を総計で０．０５～５ａｔ．％含み、残部がＡｇおよび不可避不純物からなることを特徴とする半導体装置用ボンディングワイヤ。
  2. さらにＮｉ，Ｃｕ，Ｒｈ，Ｐｄ，Ｐｔ，Ａｕの１種以上を総計で０．０１～５ａｔ．％含むことを特徴とする請求項１記載の半導体装置用ボンディングワイヤ。
  3. さらにＢｅ，Ｂ，Ｐ，Ｃａ，Ｙ，Ｌａ，Ｃｅの１種以上を総計で１０～３００ａｔ．ｐｐｍ含むことを特徴とする請求項１又は２記載の半導体装置用ボンディングワイヤ。
  4. ボンディングワイヤ表面部のＩｎ，Ｇａ，Ｃｄの総計ａｔ．％が、前記ボンディングワイヤ内部の前記総計ａｔ．％の２倍以上であることを特徴とする請求項１～３のいずれか１項記載の半導体装置用ボンディングワイヤ。
  5. ワイヤ軸に垂直方向の断面における平均結晶粒径が０．２～３．５μｍであることを特徴とする請求項１～４のいずれか１項記載の半導体装置用ボンディングワイヤ。
  6. 前記ボンディングワイヤのワイヤ軸を含むワイヤ軸に平行な断面の結晶方位を測定したときの測定結果において、前記ボンディングワイヤの長手方向に対して角度差が１５度以下である結晶方位＜１００＞の存在比率が面積率で、３０％以上１００％以下であることを特徴とする請求項１～５のいずれか１項記載の半導体装置用ボンディングワイヤ。
  7. 前記ボンディングワイヤの最表面から深さ方向に対して０～１ｎｍの領域における金属元素の総計に対するＩｎ，Ｇａ，Ｃｄの中から選ばれた少なくとも１つ以上の元素の平均総計ａｔ．％が、前記ボンディングワイヤの最表面から深さ方向に対して１～１０ｎｍの領域における該平均総計ａｔ．％の１．２倍以上であることを特徴とする請求項１～６のいずれか１項記載の半導体装置用ボンディングワイヤ。
     
PCT/JP2015/064417 2014-07-10 2015-05-20 半導体装置用ボンディングワイヤ WO2016006326A1 (ja)

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