Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/02—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape
  • B23K35/0255—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape for use in welding
  • B23K35/0261—Rods, electrodes, wires
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/02—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape
  • B23K35/0255—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape for use in welding
  • B23K35/0261—Rods, electrodes, wires
  • B23K35/0266—Rods, electrodes, wires flux-cored
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
  • B23K35/24—Selection of soldering or welding materials proper
  • B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
  • B23K35/302—Cu as the principal constituent
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
  • B23K35/24—Selection of soldering or welding materials proper
  • B23K35/32—Selection of soldering or welding materials proper with the principal constituent melting at more than 1550 degrees C
  • B23K35/322—Selection of soldering or welding materials proper with the principal constituent melting at more than 1550 degrees C a Pt-group metal as principal constituent
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/018—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of a noble metal or a noble metal alloy
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
• • H—ELECTRICITY
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Definitions

• the invention is related to a bonding wire, comprising a core with a surface, wherein the core comprises copper as a main component, and a coating layer superimposed over the surface of the core, wherein the coating layer comprises palladium as a main component, wherein the core comprises silver and phosphorus as further components, wherein the ratio between the silver content and the phosphorus content of the core is in the range of 0.03 to 2.
• the invention further relates to a system for bonding an electronic device, comprising a first bonding pad, a second bonding pad and a wire according to the invention, wherein the inventive wire is connected to at least one of the bonding pads by means of wedge- bonding.
• the invention further relates to a method for manufacturing a wire according to the invention.
• Bonding wires are used in the manufacture of semiconductor devices for electrically interconnecting an integrated circuit and a printed circuit board during semiconductor device fabrication. Further, bonding wires are used in power electronic applications to electrically connect transistors, diodes and the like with pads or pins of the housing. While bonding wires were made from gold in the beginning, nowadays less expensive materials are used such as copper. While copper wire provides very good electric and thermal conductivity, wedge-bonding of copper wire has its challenges. Moreover, copper wires are susceptible to oxidation of the wire.
• bonding wires of circular cross- section and bonding ribbons which have a more or less rectangular cross-section.
• Both types of wire geometries have their advantages making them useful for specific applications.
• both types of geometry have their share in the market.
• bonding ribbons have a larger contact area for a given cross-sectional area.
• bending of the ribbons is limited and orientation of the ribbon must be observed when bonding in order to arrive at acceptable electrical contact between the ribbon and the element to which it is bonded.
• bonding wires these are more flexible to bending.
• bonding involves either soldering or larger deformation of the wire in the bonding process, which can cause harm or even destroy the bonding pad and underlying electric structures of the element, which is bonded thereto.
• bonding wire comprises all shapes of cross-sections and all usual wire diameters, though bonding wires with circular cross-section and thin diameters are preferred.
• copper is chosen because of high electric conductivity.
• the coating layer palladium is one of the possible choices.
• wires of the present invention have been found to solve at least one of the objects mentioned above. Further, a process for manufacturing these wires has been found which overcomes at least one of the challenges of manufacturing wires. Further, systems comprising the wires of the invention were found to be more reliable at the interface between the wire according to the invention and other electrical elements, e.g., the printed circuit board, the pad/pin etc.
• a first aspect of the invention is a bonding wire, comprising:
• the core comprises copper as a main component, and a coating layer superimposed over the surface of the core,
• the coating layer comprises palladium as a main component
• the core comprises silver and phosphorus as further components, wherein the ratio between the silver content and the phosphorus content of the core is in the range of 0.03 to 2.
• the ratio of silver content to phosphorous content is in the range of 0.05 to 2, most preferred in the range of 0.05 to 0.5. If no other specific definition is provided, all contents or shares of components are presently given as shares in weight. In particular, shares given in percent are understood as weight-%, and shares given in ppm (parts per million) are understood as weight-ppm.
• the term "superimposed" in the context of this invention is used to describe the relative position of a first item, e.g. a copper core, with respect to a second item, e.g. a coating layer. Possibly, further items, such as an intermediate layer, might be arranged between the first and the second item.
• the second item is at least partially superimposed over the first item, e.g. for at least 30 %, 50 %, 70 % or for at least 90 % with respect to the total surface of the first item. Most preferably, the second item is completely superimposed over the first item.
• intermediate layer in the context of this invention is a region of the wire, between the copper core and the coating layer. In this region, material as in the core as well as material as in the coating layer is present in combination.
• the wire is a bonding wire in particular for bonding in microelectronics.
• the wire is preferably a one-piece object.
• a component is a "main component" if the share of this component exceeds all further components of a referenced material.
• a main component comprises at least 50% of the total weight of the material.
• a silver content of the core is at least 5 ppm.
• the silver content is in the range of 5 ppm to 1000 ppm.
• the silver content is in the range of 5 ppm to 100 ppm.
• the copper content of the core is at least 99.98%. This allows for using common supplies of copper material for wire bonding without exceedingly high costs due to high purity specifications. Further preferably, a total of all metal impurities of the core, apart from silver, sums up to less than 30 ppm.
• the coating layer has a thickness of less than 0.5 pm. If the coating layer is sufficiently thin, this provides for neglectable effects of the coating layer in the bonding process.
• the term "thickness" in the context of this invention is used to define the size of a layer in perpendicular direction to the longitudi- nal axis of the copper core, which layer is at least partially superimposed over the surface copper core.
• the palladium content of the layer is at least 50%, more preferably at least 95%.
• the coating layer con- sists of pure palladium with less than 1% of other components.
• those components are preferably noble metals.
• the present invention is particularly related to thin bonding wires.
• the observed effects are specifically beneficial to thin wires, for example because of the sensitivity to oxida- tion of such wires.
• the term "thin wire” is defined as a wire having a diameter in the range of 8 pm to 80 pm.
• Such thin wires mostly, but not necessarily have a cross-sectional view essentially in the shape of a circle.
• a cross-sectional view in the present context refers to a view of a cut through the wire, wherein the plane of the cut is perpendicular to the longitudinal extension of the wire.
• the cross-sectional view can be found at any position on the longitudinal extension of the wire.
• a "longest path” through the wire in a cross-section is the longest chord which can be laid through the cross-section of the wire within the plane of the cross-sectional view.
• a "shortest path” through the wire in a cross-section is the longest chord perpendicular to the longest path within the plane of the cross-sectional view defined above.
• the longest path and the shortest path become indistinguishable and share the same value.
• the term "diameter” is the arithmetic mean of all geometric diameters of any plane and in any direction, wherein all planes are perpendicular to the longitudinal extension of the wire.
• a ratio of an elongation of the wire and a diameter of the wire is in the range of 0.25 %/ ⁇ to 0.75 %/pm.
• the wire meets the relation 0.0008 ⁇ E/(d*CP) ⁇ 0.0375, wherein E is the elongation of the wire measured in %, d is the di- ameter of the wire measured in pm and CP is the phosphorus content of the wire core measured in ppm.
• E is the elongation of the wire measured in %
• d is the di- ameter of the wire measured in pm
• CP is the phosphorus content of the wire core measured in ppm.
• Such wire has an optimized phosphorous content dependent on the respective demands for its elongation and diameter. More preferably, the wire meets the relation 0.002 ⁇ E/(d * CP) ⁇ 0.02, and most preferably the wire meets the relation 0.003 ⁇ E/(d*CP) ⁇ 0.015
• the wire meets the relation 0.000025 ⁇ E/(d * CAg) ⁇ 0.15, wherein E is the elongation of the wire measured in %, d is the diameter of the wire measured in ⁇ and CAg is the silver content of the wire core measured in ppm.
• E is the elongation of the wire measured in %
• d is the diameter of the wire measured in ⁇
• CAg is the silver content of the wire core measured in ppm.
• Such wire has an optimized silver content dependent on the respective demands for its elongation and diameter.
• the wire meets the relation 0.00025 ⁇ E/(d * Ag) ⁇ 0.15, and most preferably the wire meets the relation 0.005 ⁇ E/(d*Ag) ⁇ 0.12.
• An inventive wire is preferably treated in an annealing step with a temperature of at least 370*C. Even more preferred, the temperature of the annealing step is at least 430°C, wherein higher annealing temperatures can provide for higher values for the elongation of the wire.
• annealing is done by pulling the wire through an annealing oven of a given length and with a defined temperature profile at a given speed.
• An exposure time of a thin wire to the annealing temperature is typically in the range of 0.1 second to 10 seconds.
• a hardness of the wire core is not higher than 120.00 Hv0.5. This limits the maximum forces applied on the bond pad which can occur during the automated bonding procedure. Such limitation is particularly helpful if mechanically sensitive structures are aligned below the bond pad. This is particularly true if the bond pad consists of a soft material like aluminium or gold.
• the sensitive structure can, for example, comprise one or several layers of porous silicon dioxide, in particular with a dielectric constant of less than 2.5. Such porous and hence weak material is becoming increasingly common as it can help to increase the device performance. Therefore, the mechanical properties of the inventive bonding wire are optimized to avoid cracking or other damaging of the weak layers.
• the hardness of the wire core is ideally measured at the wire as a finished product, that means after all manufacturing steps like coating and annealing have been finished.
• a metallography cross section of the wire can be prepared in order to produce a flat surface. This can be done in the longitudinal direction, which is more common, or in the transversa! direction.
• a specific resistivity of the inventive wire is in the range of 1.65 to 1.9 pOhm * cm, such conductivity being suitable for many applications.
• a more pre- ferred resistivity of the wire is in the range of 1.8 to 1.9 pOhm * cm.
• a further aspect of the invention is a system for bonding an electronic device, comprising a first bonding pad, a second bonding pad and a wire according to the invention, wherein the wire is connected to at least one of the bonding pads by means of wedge- bonding. This combination of an inventive wire in a system is preferred due to the fact that the wire has especially beneficial properties with respect to wedge bonding.
• a process window area for the at least one wedge bond has a value of at least 3500 mA*g in the case of a bonding a wire of 20 pm diameter bonded to a gold bond pad.
• a process window area for bonding wires is known in the art and is widely used to compare different wires. In principle, it is the product of a bonding window of an ultrasonic energy used in the bonding and a bonding window of a force used in the bonding, wherein the resulting bond has to meet certain pull test specifications, e.g. a pull force of 3 grams, no non-stick on lead etc..
• the actual value of the process window area of a given wire further depends on the wire diameter as well as the bond pad material.
• the scope an inventive system is not limited to wires of this diameter and bond pads made of gold, but names this data only for definition purpose.
• a yet further aspect of the invention is a method for connecting an electrical device, comprising the steps a. providing a wire according to the invention
• steps b and c are performed without the use of a forming gas.
• the wire according to the invention shows excellent properties with respect to oxidation effects. This is specifically true if a complete encapsulation of the copper core with the palladium coating layer is present in combination with a certain amount of phosphorous in the core material.
• the resulting properties allow for processing without using forming gas and hence lead to significant savings in costs and hazard precautions.
• Forming gas is known in the art as a mixture of an inert gas like nitrogen with hydrogen, wherein the hydrogen content may provide for reduction reactions of oxidized wire material.
• omitting of forming gas means that no reactive compound like hydrogen is used. Nevertheless, use of an inert gas like nitrogen can still be advantageous.
• a yet further aspect of the invention is a method for manufacturing a bonding wire according to the invention, comprising the steps a. providing a copper core precursor with the demanded contents of phospho- rus and silver;
• the manufacturing method is generally known in the art. It is pointed out that the coating layer might be applied by any known or suitable method like mechanical coating, electric plating, electroless plating, physical vapor deposition (PVD), chemical vapor deposition (CVD) and more.
• the coating can be done before or after the drawing of the wire, which may be dependent on properties of the respective coating and coating method. In particular, the coating might be performed at an intermediate step, with a drawing of the wire or precursor occurring before as well as after the coating step.
• the phosphorous might be doped into the copper core as known in the art.
• the silver might be added to the copper in a specific pro- duction step of the core material.
• the silver content might be obtained by selection of available copper with a sufficient impurity specification.
• silver is often contained in copper as an impurity, it being possible of provide the demanded core material without applying additional process steps, but by analytically controlling the raw material.
• the annealing is performed in a controlled way as known in the art in order to achieve a softening of the wire and/or optimizing the crystal structure of the wire according to the respective demands.
• the annealing is done dynamically while the wire is moved through an annealing oven and spooled onto a reel after having left the oven.
• a wire 1 is depicted.
• Figure 2 shows a cross sectional view of wire 1.
• a copper core 2 is in the middle ' of the cross sectional view.
• the copper core 2 is encompassed by a coating layer 3.
• a surface 15 of the copper core is located.
• the diameter of copper core 2 is shown as the end to end distance between the intersections of line L with the surface 15.
• the diameter of wire 1 is the end-to-end distance between the intersections of line L through the center 23 and the outer limit of wire 1.
• the thickness of coating layer 3 is depicted.
• Figure 3 shows a process for manufacturing a wire according to the invention.
• Figure 4 depicts an electric device 10 comprising two elements 1 and a wire 1.
• the wire 1 electrically connects the two elements 1.
• the dashed lines mean further connections or circuitry which connect the elements 11 with external wiring of a packaging device surrounding the elements 11.
• the elements 11 can comprise bond pads, inte- grated circuits, LEDs or the like.
• Figure 5 shows a sketch of a wire pull test. To a substrate 20, a wire 1 is bonded in bonds 21 at an angle 19 of 45°. A pull hook 17 pulls wire 1. The angle 32, which is formed when the pull hook 17 pulls the wire 1, is 90°.
• test specimen i.e. a wire of 1.0 m in length
• the voltage was recorded with a device for measuring voltage. This set-up was repeated with at least 4 test specimens. The arithmetic mean of the four measurements was used for the calculations given below.
• Bonding of a wire to a substrate plated with gold was performed at 20°C, wherein the bonding was applied to the gold surface.
• the wire was wedged with its second end to the substrate.
• the distance of the bonds between the two ends of the wire was in the range of from 5 to 20 mm. This distance was selected in order to assure the angle of 45° between the wire and the substrate.
• ultrasonic sound of a frequency in the range of 60 - 120 kHz was applied to the bondtool for 40 to 500 milliseconds.
• a quantity of copper material of at least 99.99 % purity (“4N-copper”) is molten in a crucible. Small amounts of silver (Ag) and phosphorous (P) are added to the melt and an even distribution of the added components in the copper melt is provided. The added amounts of silver and phosphorous are calculated to contribute to a share of 10 ppm Ag and 75 ppm P. Then a wire core precursor is cast from the melt.
• the wire core precursor is then drawn in in several drawing steps to form the wire core 2 with a diameter of 20 ym.
• the cross section of the wire core 2 is of essentially circular shape. It is to be understood that the wire diameter is not considered to be a highly exact value due to fluctuations in the shape of the cross section, a thickness of the coating layer or the like. If a wire is presently defined to have a diameter of e.g. 20 pm, the diameter is understood to be in the range of 19.5 to 20.5 pm.
• the wire core is then coated with a layer 3 consisting of Palladium (Pd) of at least 99% purity.
• the thickness of the coating layer 3 is in the range of 100 nm or less. Therefore, the coating layer is not significantly adding to the wire diameter.
• the coated wire is then annealed in an annealing step in order to further adjust parameters like elongation, hardness, crystal structures and the like.
• the annealing is performed dynamically by running the wire through an annealing oven of a defined length and temperature with a defined speed. After leaving the oven, the wire is spooled on reels for packaging.
• the annealing time which is the time during which a given piece of the moving wire remains in the heated oven, is about 0.76 s.
• Table 1 and table 2 show data of several examples IW1 to IW5 of wires according to the invention.
• the exemplary inventive wires have been produced identically, let alone for exposure to different annealing temperatures in the annealing step. None of the further annealing parameters apart from the temperature have been changed.
• the inventive wires generally show a high value of the second bond window area, i.e. 4000 or more.
• the wire Comp2 shows also a high second bond window area, but it has high standard deviation on the stitch pull value, which is disadvantageous for the process stability.
• the Comp2 sample has a significantly bad value of the ball shape.
• the wire Comp2 would not be chosen at least for ball bonding applications when compared to the inventive wires.
• the second bond window area is significantly lower than for any of the inventive wires.
• inventive wires particularly ad- vantageous for second bond or wedge bond applications.
• inventive wires can be a pre- ferred choice for wedge-wedge-bonding applications, but also show excellent results for ball-wedge-bonding applications.
• the example IW5 has some striking properties, e.g. the elongation value and/or the wire hardness value. This owes to the fact that this wire has been annealed at a rather low temperature of 400 °C. Softening effects from the annealing process are comparably weak in this range. On the other hand, such wire can be advantageous in specific applications, e.g. when a particularly good ball shape or a high break load is demanded for.
• IW1 to IW4 are specifically preferred examples of inventive wires.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Computer Hardware Design (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Power Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Wire Bonding (AREA)

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Citations (5)

• 2013
  • 2013-03-05 SG SG2013016415A patent/SG2013016415A/en unknown
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